The Eldar with head swap is probaby your best bet I've never seen it done for Van Saar, but I've seen it done for imperial assasins and it looks pretty good. If you can find some of the old metal eldar guardians, they're a bit smaller and will work as juves.
There are other viable options out there, but it's hard to find enough different poses to make a full gang.
The Alpha forge line has some that are pretty good futuristic types in body suits.
http://freeboardgamesnow.com/sciencefictionsalvagecrew.aspx
Also, the "Gangs of MEga City One" plastics that come with the boxed set might work. THey're not quite as techy as Van Saar, but they have potential, and they are usually quite cheap.
http://www.dakkadakka.com/core/gallery-viewimage.jsp?i=127201
The whole game box with 12 figs is on ebay for less than 20 bucks
Lastly, the Star Wars miniatures game is packed with sci-fi figs in bodysuits/flightsuits. The problem there is that they are tall enough, but not "heroic" in proportion so swaping in current version
40k weapons will not look great. Still, they're dirt cheap, easy to paint over, and if you look long enough you can probably find enough appropriate figures with the weapons you want.