The AW-70/71 automatic trannies in the 240, 740, 940's are about as bulletproof as slushboxes get. When I sold my 90 240 at 300k it still had its original tranny and engine. Neither had ever been out of the car or had major work (head gasket etc.). These trannies, if maintained can outlast the car. Personally, I've read about more problems on this board with some of the manual trannies in 240's than slushboxes.

If you get serious about buying a slushbox here are some things to look for and consider:

1. Check for leaks at the tranny cooling lines where the line is bracketed to the frame. The bracket can gradually crack the lines.
2. Also check the rear tail housing area. The tailshaft seal and its donut can also loosen over time and leak.
3. No slips!
4. I'd flush the tranny yourself and refill. Use synthetic ATF if the AW-70/71's are ok with synthetic. Personally, I've had terrific luck with synthetic tranny fluid in both manuals and automatics but I can't remember if synthetic is compatible with that transmission.