I just bought a nice body 126 300SD with a junk motor & I had a parts car 123 with 320,000 miles on it & I thought the motor would be tired. After warming it up I found the compression to be 300 psi in every cylinder. Many people would be very gun shy about putting a engine with this many miles in but since Im doing it myself on slow days I did it & it runs GREAT!

If your doing the work yourself & you dont mind the work go for it. But I would not advise hiring out this work. Also dont get caught up in the trap of replaceing all these items just because "your in there anyways" If the motor mounts are still good leave them alone or take the better set from the 2 cars. Some like to buy $500-$1000 worth of new parts when doing this (ie oil cooler hoses, mounts, tranny lines, water pump,etc) Just replace what is bad if any. Although if the oil filter housing is leaking this is the time to do that.