I wouldn't bother with anything else than Doom³ and maybe, if you like it a lot, its expansion. The other ones are nice for nostalgia but I think you'll find that they become a bore after a while...and if you're already short on time, better not to waste the money on it.
Doom³ is actually a pretty good game if you can get over the cheesy scares it gives you. I don't mind - I like scary games - but I know a few people who were 'not amused' by doors sliding open behind them and monsters spilling in from random portals.

Wusses.
Graphics are still in the upper range, even today.
EDIT: offer ends Monday, September 8? Wtf?