If you're after something that reasonably resembles the original four suit types, I think you're still looking at a spend of about £6-7 each on average. If you buy plastic GW bits from eBay, a bits-seller, or you or a friend have some left-overs, then here are some suggestions:
Orrus
- Tau Stealth suit body
- Space Marine Terminator legs
- Space Marine Terminator Lightning claws
- Space Marine Scout head
This is pretty self-explanatory, for the most part. If you cut the claws off the arms, what's left looks like a bolt launchers on the back of the power fists, especially if you use a small drill to drill out a row of little holes.
Malcadon
- Dark Eldar Kabalite Warrior body and legs
- Dark Eldar Wych arms with claws
- Dark Eldar Wych, Hellion, or Jetbike head (the blank-faceplate ones look good, but the half-masks are nice too)
Again, pretty self-explanatory. This looks more convincing if you add some greenstuff or wire cables from the claws, and you can also use the Orrus claws you snipped off to make the Malcadon's bigger if you wish.
Yeld
- Dark Eldar Scourge or Eldar Swooping Hawk
These are pretty much good as-is. I would go for the Scourge personally, but use some smaller wings (perhaps from a Swooping Hawk). I don't think that holding a rifle rather than having guns built into the gauntlets is a big issue either, but you could look at using some Eldar Dire Avenger Exarch arms if you prefer the style - just cut the ammo clips off and connect the guns back to the rear of the model with some wire/greenstuff cables.
Jakara
- Dark Eldar Wych, plastic Eldar Dire Avenger Exarch, or Eldar Howling Banshee
These are again pretty much good to go as-is, although a head-swap will help sell the model better as Jakara.
The plastic Dire Avenger Exarch has a sword and a shield arm options that are basically perfect, and this would be my choice. The arms are easy to find, so you could also just use an Eldar Guardian body and the Exarch arms as a cheaper alternative.
Of course, this is using GW plastic models, and is intended to give you a recognisable analogue to the old models. If you are happy to just use a straight counts-as, then you can also be quite imaginative:
- Eldar Aspect Warriors can be used pretty much as-is, with only the Orrus requiring a less perfect fit (Striking Scorpion Exarch with power fist and shuriken pistol probably).
- Dark Eldar Gladiator/Wych cult again can be used as-is apart from the Orrus.
- Harlequins (plastic, so you could convert up a team easily enough)
- Tyranids (plastic, you could convert up a gaunt-based team easily enough)
- Necrons (plastic, you could convert up a team easily enough)
- Predator miniatures (lots of options here, whether Heroclix or Copplestone castings)
- Infinity miniatures (lots of options, but not especially cheap)
- Void or Urban War (some conversion may be required)
Ultimately I suppose it depends, on a few things: what's your spend limit, do you mind converting or kitbashing, do you want plastic or metal, do you mind counts-as, do you already have something suitable to use as a base?