I have a 2000yr model 3000 with a T444E and MT643.
It could be very likely a 2001 has a T444E also. If so, make sure you know what HP rating it has. They go from 190 - 250. The 230/250 hp versions have high torque turbos.
Per the tranny, mine is a four speed non-electronic, which is fine for pulling kids around town, but not so much for the freeway. It could be that there is an electronic 5 speed in a 2001, and if so, there could also be a 6th gear that could be turned on in the computer parameters. The other reasons for more gears is it gives you a tighter power range so you can climb with less stress on the tranny (and you) and better MPG.
Finally, knowing what rear end in it may be important depending on the tranny. Mine was a 5.11 ratio and I was pulling 2400+ rpm at 60mph. No bueno. So, I put in a 4.10 rear end. Why, because in a RE bus, swapping out from a mechanical to an electronic requires a really long and expensive harness.
If you can cruise it at 65 at 2000rpm or less, then I'd say you're good for that part. Find a good highway grade 5-6% and see how it pulls and if needed downshifts during the pull. I didn't do either of these and I ended up spending $2500 for the rear end when I only paid $2300 for the whole bus.
Be aware, don't rely on IC dealers for much support. IC corp has the dealers under their thumb when it comes to changing parameters on their stuff. Best to buy an aftermarket connector and software to connect to your ECM/TCM.
Body wise, check underneath really well for signs of rust. If it has outside storage bins that's a big bonus. Check for signs of leaking. If you have a diesel place you can get to check it out, it would be worth it to get a professional opinion.
Best of luck.