I would recommend NOT using Acme for a plasma table (at least the X and Y - the Z is probably OK). My actual CNC plasma experience is limited to about 20 mins of actual cutting time (mainly have been using my recently home built table in router configuration)... but speed is important for plasma. Greater than 100 IPM will probably be necessary for anything thinner than 1/8" (others will probably chime in with better data, or you can easily search). My table uses belt drive ("L" series timing belt), rack and pinion is probably the most popular, some use a chain drive (like bicycle chain - this may be the cheapest route for you).
One thing I have found in my brief experience is that plasma is MESSY (metal grit that blows through, as well as gets in the air and covers EVERYTHING around. I would recommend using it outside or with a water table... the water table is on my project list before I do any more plasma cutting! Your linear rails look nice (mine uses similar ones, 20mm THK on Y and 15mm Hiwin on X) - be careful to keep them clean. Knowing what I know now about the mess, if I was planning to do mainly plasma (vs router) I would use V-rails. But, the linear rails were easier to work with (using similar extrusion for X/gantry axis). And, you have them available.
On the motors, I would recommend 3 - one on the gantry, and one on each end. Mach3 can "slave" the two motors on the ends of the gantry and based on your limit switch setup can actually "square" the gantry - really cool! Or, at least couple across the gantry (belt, etc) and drive both sides.
I wish you the best of luck on your project... I am currently a college student too (U of Cinci). And I actually have a class starting in 3 mins! So I need to get going.
I think you will really enjoy Baja - I had a lot of friends who did it at Letourneau Univ (I was there a few yrs ago), and did quite well. It is a lot of work but a lot of fun. I used to hang out down there a lot, I was actually on the Formula SAE team. We used to give the Baja guys a hard time about how they needed our engine! I did take my dirt bike out a few times when they drove the car and hung out. We all wished we would have had ANY CNC equipment! :-) But I did enjoy the bridgeports and nice big lathes... just stuck with smaller mill and lathe in a home shop now.
Let me know if you need anything...