Well seen has your talking IPM then I'll guess you have been looking around the Zone too much.!!

Your on the right track with Fixed gantry and while I'm not a fan of those round type rails they fit your budget. Personaly I'd spend that bit more and buy Profiled rails but they'll do the job.

Where you start to leave the track is with 5mm pitch screws. Go for 10mm they will give far better performance and flexibilty and still have more than enough resolution for what you want. 2500mm is way too slow for most soft material and you'll want double that for some. 10mm will do every thing you need.

Next is the electronics. DONT buy that Kit it's cheap rubbish. I also don't believe the Spec they are queting because 1.8mh on 4 wire motor this size is unusual. The drives are Ripp offs Leadshine Analog drives. Analog drives are dinosaurs and history, Digital drives are so much better and don't cost fortune. Look for Leadshine DM556 these are great drives for not lot of money.

With motors Don't pay too much attention to the Chinese torque graphs because they are unrealistic jokes and mean very little in real world. The mains things to look for are low inductance and ideally can be wired Bi-polar parallel.

Also Voltage you want about 44Vdc with 50V drive rather than 36v. Steppers get there speed for volts and too some degree higher torque.

Regards the Z axis then really only you can answer whether you have enough clearence or not. No point having loads of clearence if not using because it weakens the machine. It would be easy enough to raise clearence and to little degree stiffen up by having rails on front plate.

Design wise your close and will be strong little machine but don't cripple it with low spec electronics.