You are right that 4 axle units are harder to find. I have noticed that Guilford tries to find a large fleet of units to buy cheap. This was so on the ex CR, PC, NYC GP40, and the ex NS GP35 fleets. After aquiring the units, the best ones are put into service. The worst ones are scrapped for parts, and the so-so units are rebuilt as time permits.
MEC407 said that in 10-15 years maybe Santa Fe will start retiring/selling their GP60's...
In talking with some other railfans, I've noticed that GP60's are mostly built for high speed service, not slow trains. Mostly, GRS's speed limits on their lines are 30-40, but with crappy track and all, we're looking for a unit that can go anywhere and do anything--that includes slow pulls. I believe these units can do that, but most likely will have problems.
Take the VRS GP60's...I've heard they spend lots of time sidelined because of wheel slip issues from lugging trains over the mountain up in Vermont. Would a GP60 even be a remotely good way to go?