Too broad of a statement. The D70 is being replaced by the D80 and the D50 by the D40. You can still find the D70 and D50 around though as many shops still have stock on hand. All four cameras are very capable and I have tested 3 of them out myself (haven't touched the D40 yet). In the right hands, it won't really matter which you go with.
Only buy a used camera in person and from a trusted source. I've sold off some used gear to friends and I cut them pretty hefty deals so if you know someone well enough, don't discount used cameras out of your equation. Just be aware of what you're getting into and if you're not sure, then best not to go through with it.
DSLR shutter life will differ depending on make and model. The consumer cameras, such as the D40, D50, D70, and D80 have a shutter life of around 50k actuations. The prosumer/semi-pro camera such as the D200 has a shutter life of 100k. The D2H/D2Hs and D2X/D2Xs has a shutter life of approximately 150k. Each camera is slightly different and YMMV, but those are pretty close approximations.