if your going to take the time to pull the motor apart, why not replace the bearings, have the cylinders honed, and get some lower compression forged pistons in the process? it would sure as hell save you a ton of time down the road when you wanted to up the boost/replace the turbo with a larger one.

mileage really isnt that big of a factor. its how well the motor was taken care of. ive seen motors with over 200k on them run 7lbs of boost reliably with no problems, but they were maintained before being boosted.

prices vary from place to place. youll have to check those out yourself and obviously, it will be cheaper if you do the work and install yourself.

shaftplay is the play in the center cartridge in the turbo between the bearings. any more than a fraction of an inch and you need a rebuild, or a new turbo.

hope that helps some.