Im considering getting a 2000-2003
3.0 engine from a wrecking yard and putting it in the m/s whos engine dumped .

I plan on driving it 50-100K miles over the next 5 or 10 years.

That means Im going to have to locate an engine with 100k approx

Since CR of each cylinder is all I'll know about the engine
,how can i use mileage and CR to guestimate how long it should last. Remember,this is all the info I can know before deciding to purchase it.
BTW what is the CR new?

 

I don't think you can accurately estimate (or gestimate) engine mileage based on compression ratio. It will all depend how well services were performed on the engine, if the engine was run constantly at high RPM, towing, etc. If you can open up the drain pan and valve covers - look at the condition of the bottom and top end of the engine. Getting a used engine from a wrecking yard is like buying a cat in a bag, but sometimes that is the only option and you may get lucky too.