The reason why i opted for the 220 is because running cost, road tax etc. was all cheaper hence why...but i guess if you do low miles, then the 250 will do the job nice.
the IS250s don't have timing belts, but are chain driven. Like buying any car, i would suggest get one with full service history, try and get lower miles and most importantly, ensure you negotiate hard!
Remember, the road tax and running costs are higher than average, so this puts off a lot of people, but gives you more better buying power. Always knock off 10% and try and stick to it :) Buying privately is quite good, it allows you to know as much about the car and owner so you get a vibe of the way the car has been maintained and driven.