Or you could try HeresyOnline -
http://www.heresy-online.net/forums/index.phpDaeothar's right - a 5000pt army is huge.
You're looking at about 150 marines and a dozen vehicles, at a guess.
Or more tanks and fewer troops, or more troops and fewer tanks.
I think I'd
start with 6 tactical squads, 2 assault squads and 2 devastator squads, 8 rhinos (one for each squad except the assault squads), a razorback and a 5-man command squad for a captain and retinue - this gets you an entire battle company (2nd Company is popular, the ones with the gold trim to their shoulders) which is an Apocalypse formation in its own right.
After that some tanks and maybe some skimmers or bikes, some Terminators and Scouts and you might be approaching 5000pts.
Probably, a much sensible way of looking at it would be to find out what your cousin (who's going to give you his old minis?) actually has.