New to Cnc

[JAZZCNC]

Nope don't do that either.? Rack and pinion again needs gearing with a ratio of between 2:1 and 3:1 and special attention to keeping pinion engaged into rack, so it's actually much more fiddly. It's also very inefficient compared to ballscrews with much higher backlash.
R&P is really only suitable for larger machines where fitting long ballscrews becomes troublesome, it's more suited to wood based machine with lower resolution.

Regards welding the rails then again wouldn't do that has you'll need adjustment so bolting allows shims to be inserted. Also the heat , even low heat, will affect accuracy and possibly warp or twist the rails.