Otherwise known as the Asuryani, the Craftworld Eldar have lots of choices for models.
The Eldar’s core options are for Rangers, Dire Avengers, Guardian Defenders and Storm Guardians.
The best buy for a compete Eldar kill team would be a Dire Avengers box and a Guardians box. This will cost £47.50 retail. The two boxes together will give you 5 dire avengers, including an exarch, 10 guardians and a heavy weapon platform.
The models in the Guardians box can only be built as defenders. However there is a sword and pistol in the Dire Avengers box that you could use to make one of them into a storm guardian. The sword and pistol is for the exarch but he has other options. The extra heads and other bits in the Dire Avengers box could allow you to convert one of the guardians into an extra dire avenger.
Rangers are not too great in Kill Team and you would not really want to take more than two. For this reason the Rangers box is not a great buy for Kill Team as it has 5 rangers for £25 and is cast in the Finecast material rather than plastic. Finecast is like resin but weaker.
If you want any rangers the best bet is to look for older metal sculpts on Ebay. It will be cheaper, the models stronger and you can probably get them in smaller numbers.
There is an upgrade kit for making guardian defenders into storm guardians. It will cost £11 but it gives enough options to trick out 8 guardians as storm guardians and includes some useful special weapons, the flamer and fusion gun. Be aware the upgrade bits are Finecast also.
There are oldhammer options for Dire Avengers and Guardians but honestly they are not great sculpts at all. The current Dire Avengers plastic kit is far superior. You might consider some of the old metal guardian models because while they are ugly the plastic kit is not much better. It can be a cheaper way to get some storm guardians too, since you can do without the upgrade kit.
The appearance of the oldest striking scorpion models are close enough to storm guardians to make a fair proxy too. The sculpts are fairly nice too.