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There are a lot of things that confuse me in this guide. I'll just go down the list.

1. Bloodstone? I don't see the point in this item at all on an offensive-oriented Drow Ranger. The item is best on squishier INT heroes like Skywrath Mage or Zeus who constantly spam spells. I have never seen a Drow Ranger who uses enough mana to ever warrant one of these, and there are waaaaayyyy better items for the huge amount of gold. There are perks, such as the general beefiness you get from it and the pocket suicide, but it's just not that great on Drow.

2. Mekansm. I'm aware that it's a great early item to pick up on heroes like Viper and Razor, but I don't think it fits quite as well on Drow. Those two heroes are geared for the mid game more than Drow, and have to have a survivability item to take part in early fights. It used to be Vanguard until people realized that item is awful, so now people take Mek. Viper and Razor are also going to be in the middle of fights more often than not, whereas Drow stays back and blasts away while others tank. The Mekansm needs to be in the midst of the action so everyone benefits from it.
-That being said, Mek is rarely a bad item on anyone. It's just not fitting on an offensive playstyle which appears to be aimed at late game.

3. A support should be purchasing the Animal Courier. Only buy it if nobody else does and has already spent all their gold.

4. Blink Dagger is usually better than Force Staff on Trax. The positioning is a lot more precise, and the cooldown isn't so harsh. Force is still pretty useful, though, so it depends on the match and the player's preference I think. I would at least give Blink a mention.

5. Helm of the Dominator and later Satanic are very, very good on Drow. She already has great amounts of armor with her passive, and putting some HP behind it makes you a lot tougher.

6. Manta Style (or lack thereof). Just a really good item in general. It gives you some defense with spell dodging, and also some nice stats + illusions for pushing.

7. Black King Bar. Every carry needs this at some point, even Lifestealer who has built in magic immunity.

8. Drow Ranger is not, by any means, a mobile hero. That's just plain misinformation. Her speed is the average 300, but she has no escape mechanism. Gust is decent, but only if you hit everyone. And what if Centaur Warrunner blinks in on you from the fog and stuns you while the team catches up? What if enemies come from more than one direction? If all parties are running in a straight line and you gust them backwards and stick them for a 6 second silence, you're probably in the clear. Dota is never that simple, though.

A lot of these things aren't really bad ideas in general, it's just that you say it's an offensive carry guide to Drow and the items don't fit with that description. This build is much more similar to a midgame carry or utility Drow. I think it just needs some clarification.

Eh what? viper? Bloodstone?

This is quite a new concept as seen on Viper, but I dont really understand the Mekanism on her, I suggest explaining how or in what situation it is used. Same as for Bloodstone. Id also suggest a more detailed explanation on split pushing builds/ ganking or offensive ones. I would as well suggest adding a Black King Bar to your build, since you probably will not silence all the heroes.
I see potential in this guide, just try to explain your ideas a bit better :D

Actually had not thought of that, thanks a lot, will probably replace it tomorrow, its quitting time at work again so cant do it now

Protip: remove the desolator and urn for a medalion of courage

The force staff would be a great idea for the alternative build as well instead of using blink dagger, deffinatly do die a lot on this build, however i still manage a 3/1 k/d ratio on avarage. The idea of this build is to pull agro off of other people by forcing them to push against you and then falling back to gank them. I was incredibly sceptical of getting Desolator myself until i tryd it and realized its possible to use much more effectivly than a lot of other UAM's. I did state that the Assault Cuirass could be replaced with pretty much anything, seeing as at that point you have an insanly fast attack speed anyway and are killing relativly fast. And much agreed that there are many ways to set up a team. I personally love comboing the Drow Ranger with the Windranger. Will whip up another alternative build set for this guide.

The Frost Arrows will go off when activated instead of desolators armor reduction, however since it is a active skill it functions to have a desolator althou difficult to use to full potential. A desolator will reduce armor for a duration, during which you can fire a frost arrow and not lose a single bit of dps. The Sniper does make for a better potential carry, due to his longer attack range and Headshot however, the Drow Ranger is a much better option to gank with and can then gain gold much faster than sniper. This is generally noticable after about 25 to 30 minutes into a game, as having 20,000 gold of gear opposed to about 17,000 time it takes for me to generally kill a tower or base using drow ranger is about that of a sniper of similar level.
However the solo potential of this build mainly involves the long attack range, surpassing sniper until he has a rank of Take Aim. And by far surpasing Luna although overall DOTA 2 is an incredibly well balanced game and in the end it all comes down to strategy and playstyle, as each character can help in one way or another.

If people really want mobility on Drow Ranger, why not consider a build of Power Treads + Force Staff + Mekansm? The idea of this guide is to pump Drow up for some mid game power, and the build above only cost about ~5k gold, and gives mobility, survivability, stats (more mana to work with the mek active). With Yasha or an Mask of Madness, Drow still cannot fight effectively unless your positioning is very good, and need another big item (like Black King Bar), and even then the enemies will get close and kill you if they really want to.
Now, some people would argue you lack damage, which I perfectly agree. But fixing this would be easy, you get another damage dealer on you team, running a dual or tri core strats instead of a 4 protect 1 strats.

Frost Arrows will go first because it's an ability UAM, so it has priority on items.

Which UAM goes first if you have Desolater and Frost Arrows? I believe it will be Frost Arrows, but I'm not a fan of having a Desolator on her. Also, she benefits from mobility more than durability - she and sniper are basically the same hero though Sniper has more carry potential (via headshot procs).

Drow's advantage is having a ranged carry and support which she is increasing the damage of, simply by being alive. I don't think she's strong enough as a 1 position compared to say Luna.

while the Drum of Endurance is a nice item I do not think it provides quite as much protection as the Mekansm simply because it is a single click instant heal so if you are not killed in a single stun then you can add 250 HP but just have to be careful of your mana use. Although the drum would be a better replacement for the Urn of Shadows

I think if you want the extra durability from mekansm, a drum might be a better choice IMO. Drum gives you movespeed and great all around stats.