Engine Life
Over the years i've had 6 engine rebuilds done here in the UK following TBO + 20%. The general comment that i'm given by those who do this work is that engines that are well in excess of 3000 hours before rebuild are common.
The 3000 hour + engine, when they have been properly maintained, are usually in good condition and need no more work than a TBO compliant engine. Norvick have a terrible reputation for taking forever with 3-4 months not being unusual. I've had engines turned round in a little over a week so Norvick and others like them must have engines sat on the shelf for most of the time. I have yet to pay in excess of £9000 including vat for a rebuild but hear of horror stories well in excess of that.
All the engines that I have had rebuilt have completed their TBO+ afterward with only minor unscheduled maintenance, usually to cylinders. Even cylinder maintenance was reduced when I changed over to Superior cylinders. I understand that Lycoming and Continental have got their act together at long last, with regard to their cylinders, which are now more likely to last the TBO period than they did at one time.
The 12 year TBO period will be very important should you be importing the aircraft into EASA land. EASA, I understand, will not accept an engine over 12years old onto the register.
With regard to purchase. The engine life must be on the side of the buyer. It is the buyer that will foot any bills later. For a standard 4 cylinder engine I would budget £6 pounds per hour for engine rebuild/replacement. Value your proposed purchase assuming a zero hours engine and reduce the purchase price by £6 per hour engine time flown. For any aircaft with an engine over its TBO reduce the purchase price by the full cost of a rebuild/replacement.