Can anyone give me advise. What is the best volvo to buy which will last a long time and be trouble free as to engine and trans issues. I currently have a 2001 v70 XC. 160k and already had to replace the rear differ since it was squealing. My private mech. told me this is VERY unusual for this to happen.

 

Since you leave it open to any model or year I have no idea but I can tell you that ANY 4 wd car is going to have more issues than a 2 wd car - especially one that old.
(As an aside, my Volvo mechanic specifically advised me against the V70 XC citing the number he see's in his shop when I drove a low mileage metallic green beauty into his shop for a pre-buy check. I ended up passing on it because my wife didn't want 3 Volvos but I've always wondered....)

 

There is no answer to this. A well maintained one is the best one. I've seen people too cheap to change the oil cause major damage in 30k miles. I would take a high mileage, well maintained one over a low mileage neglected one any day.