Sproink!

Yes Guinsoo's was the original name of Scythe of Vyse.

Any hero with low cool downs or a lot of spells, should have a Linken's Sphere as an optional core item. Heroes like Weaver, Storm Spirit and Akasha all have it as an optional core item, Enchantress is he same way in my opinion, just nobody ever does it because like you said, it's expensive. There are quite a number of team compositions that are popular and even used in competitive games, that can get wrecked by a few heroes getting Linken's Sphere. Great stats, great regeneration and even some good damage with a great effect. Just overlooked more than often.

While I agree that an Aghanim's Scepter is a must on Enchantress, sometimes putting off one or two more pieces for a Linken's Sphere is what she needs. If you ever played her more than a handful of times, you should know what I mean when I refer to these situations.

Vanguard is nice optional iteam on her because combined with untoucheble,if they got a lot melee heroes. But i'm not sure about linkens sphere this item should be taken for her very rare, it's very costy and can cost you a lose because for that money you could bought something better for the team. Usually you just won't have that spare money for it.

A lot of Enchantress players always complain about her survivability and strength gain, yet I have only seen a few build Vanguard or Linken Sphere on her. In a game against ranged auto attackers or heavy nukes/disables, they are both great items for her that can be switched in/out with her. I am glad that you do recommend Urn of Shadows on her though, it makes jungling between ganks much faster and you don't have to worry about your mana too much.