Batrider - Napalm Death

Yeah that would be my feeling for a build order too Peppo, you really need boots before you can gank (otherwise Firefly is difficult to use effectively) unless you're lucky enough to get a haste rune. The Magic Wand is very useful for those closely fought ganks though. I do like Urn of Shadows on him too, although you have to balance that against how quickly you need that Blink Dagger - general rule being whether you're solo ganking and/or whether your team has decent stuns/slows to help you get a Flaming Lasso.

Eul's is useful for the movement speed and the extra disable, but I'd always pick up a Force Staff first. Linken's Sphere I can see the attraction, but for the price think Shiva's Guard or Pipe of Insight would be better team choices.

I wanted to add that pro players usually buy items in the following order: Bottle, Boots of Speed, Magic Wand, Blink Dagger, Force Staff, Boots of Travel (disassembling the Tranquils if you have any) and extensions. I saw some players going for an Eul's Scepter of Divinity (which is in your guide already) as extension or even Mask of Madness to maximize the movement speed when lassoing; even though I still prefer BKB or Shiva's. Linken's Sphere is used a lot in pubs too but I've never seen it as a pickup in competitive games.

Thanks for the feedback guys - I've moved a few items around a bit as suggested. I always like to have a Force Staff in game, but wasn't sure if it should be considered core if your team has other initiators.

Boots are open to debate - I've tried everything except Phase Boots and found all of them useful, including Tranquil Boots, although I do worry a little about them breaking mid-gank and leaving you vulnerable.

Rod of Atos probably worth considering as it's good value for money - although I often feel "wish I'd bought a sheepstick". Bloodstone you don't need the mana regen so much, but the HP is useful, and the rapid respawn VERY useful, as Bat does tend to build up charges well.

Can see the potential of a 2 sticky/max firefly build - it guarantees more damage on ganks where you don't usually have time to get a load of stacks on them, although you do lose some potency on your right clicks and any use of Flamebreak.

As Peppo said tranquils are a good choice for a situational item.
You might wanna put an atos up there instead of the bloodstone as a bottle covers most of your mana costs.

The Force Staff is definitely a core item, not an extension. Aside from that, +1, even though some players are testing different skill builds such as two points in Sticky Napalm or max Flamebreak at level 10. Also, you may want to consider Tranquil Boots as a situational item.