The only way to do a prebuy on a Sabreliner is a 48 month inspection.
This is the biggy. But once it is opened up and if it's bad it will not fly again, unless you want to spend some major $$. Expect to spend $75,000 to $100,000 on the average 48 month.
If the 60 model does not have an AP105 autopilot or better, then rvsm will deff be cost prohibitive. Also be wary of your engine times on the JT12. Alot of them are advertised with mid time engines however the compressor and turbine disc will time time out before the o/h is due, creating what we call a short run engine.
Sabre 75's are much the same with the exception of the avionics and engines. Most of them have an AP105 which is rvsm capable. Sabreliner is working on the stc now for the 75 model with hopes of stc'ing the 40/60 next summer. Avmats has told me support for the CF700 is good from GE and it can be made stage 3 for around $200,000.
I think you mentioned that the 75 was a 1972 model. Might want to check that, I believe they didn't come out until 1974.
For Sabre maintenance go to Avmats, Jet Harbor or Sabre Corp.
Have flow the 75/80, 60 and currently fly one of the best equipped 40A's in the country. They are good flying planes, even easy to hand fly at altitude. Real popular down south also.