For a really close match, the Cannon Fodder lack combat armor, but I don't mind - I think they make fine Space Marines as-is!
It's pricey, but it looks to me like the Warhammer 40K "Valkyrie" makes a fair dropship analogue.
I just wish there were some better APC options out there! (The APC from the original Iron-Core line seems like a great match, but it's currently out of print, with no public annoucement that I know of of any plans for Wargames Atlantic to reprint it. One could hope, though!)
I think a modern Boxer IFV scale model would make a great Sci-fi APC, if only I could find one in a close enough scale!
Some sentry guns and the like would be nice as well - perhaps some of the items from Grognard Command and Accessory set might be improvised into something appropriate? (Archon Studio's "Dungeons and Lasers" scatter terrain sets do include some close-enough setnrygun models, at least, in addition to a few other great sci-fi bits such as some robots that look a bit like some great little not-Daleks.)
Reaper has some great plastic alien models (Reaper Bones Kulathi and Bathalians) which would make nice, inexpensive "not-xenomorphs", while Wargames Atlantic's giant spiders make fine not-StarshipTroopersBugs! (or not-Zergs?)
Sounds like a fun project - hopefully you'll post pictures :)