I have a xr works myself, and it still works good. The diff gears are the same, all the drivetrain components sans the driveshafts are the same between the mbx4 and mbx5. The only other difference is the rear end gears in the xr, the conical gear is 11 teeth instead of 13 and the diff gear is 40 instead of 38, I think.
Arms are not to difficult to find, but if you ever get the chance to get some, do it. I also found out that the front shock tower can can go bad over time, the holes where the hinge pins pass through will open up over time, especially if you tend to crash alot. When the holes get larger, this creates lots of unwanted slop and will actually change the caster as the cars is on track, creating an inconsistant handling car.