Those Pittman motors are way more powerful than one would expect, they would run the mill fine. Its a shame we lost Aaron as the owner of IH, IMHO the people lost a lot of what Aaron was trying to do for them when IH was sold. Its my understanding the company has again been sold, hopefully they will bring the company back to a level more in line with what it was when Aaron was running it.
I have this exact mill, mine was built in 2001 (same color also). I have been pleased with mine all though a little trouble with the Z axis. The motor on mine has held up great, in fact I feel like it may be a little better than most Asian motors that come with many of the mills these days. Both power and durability have been very good with it. On the Z axis the dovetails have spread a bit at the bottom of the travel ( casting seasoning maybe?). Its not bad, but something has moved in the casting to cause it and it gets tight at the bottom of the travel.
One problem I did have was every bearing in the head that was above the oil line died with in hours of breaking in. They had drawn moisture and rusted. Mine set in storage for many years also so this may have been the reason.
I think the Y axis nut mount will be different than the mount supplied by Aaron, and maybe even the X axis. The Z axis mod Aaron outlined to fix slop in the will need to be dune the same as the IH mill (maybe some small differences).
Also every gib in the machine may really need help. They looked like you took a grinder to them on mine, but after making them flat/fit the machine worked so much better. Movement, accuracy and slop where all improved a great deal and the machine has shown no signs of wear in a few years of use. No lapping/scraping on the dovetails, just the work done on the gibs.
Over all I feel these are good castings/machines to start with. Maybe no better than others, but certainly no worse. BTW be careful when moving the head around, they use filler to smooth the sides of the head before painting. It will chip and come out in chucks if you let something dig into it. The paint has held up very good on mine (where I didn't crack the filler) so if you can keep from chipping when doing the CNC mods it will be worth being careful with the painted surfaces.
IDK if your going to go with ball screws, if not the factory nuts and end mounts are possibly the worst part of this machine. New nuts and end bearings would fix this, but with Aarons kit you will at least have the end bearings covered. The factory nuts do have backlash adjustment, but it leaves it to stiff for hand cranks, but may work ok with the servos. Ball screws would be best.
Jess
GOD Bless, and prayers for all.