As a newbie and on your student budget get a road bike instead of a tri bike to start. If you can get your size an model nailed down properly there are lots of good second hand deals on ebay. As a mountain biker you probably have some fairly decent biking skills, but a road bike is still far more versatile than a tri bike. Get a tri bike after you graduate if you've decided that you like tri enough to stick with it.

Different manufacturers do size differently so do be careful, but there is still a lot of consistancy. IMO, you should be more concerned with top tube length than seat post length although bikes are measured by their seat post.

Here's an excellent article to get you started:

http://www.coloradocyclist.com/bikefit/

I found the online fit calculator at www.competitivecyclist.com has anybody used this? I'm 5'11 with a 34" inseam (floor to crotch) and it recomends a seat tube (centre to centre) of 56.8-57.3 cm and a top tube length of 58.6-59 cm from what I've seen this means that I should be looking for a 58-60 cm bike, does this sound reasonable?