I take it you are having an engineer survey it for you, can you not simply say he has found xyz wrong and the price does not reflect it.. and get him to drop it accordingly? or have him rectify it before sale, after all another survey would pick up the same things so he would be in the same boat again...
you can find the information as to engine overhaul intervals requirements UK wise in here you are now allowed an extension on the time period of 20% @ the engineers discretion
http://www.caa.co.uk/docs/33/CAP747.PDF
see appendix 1 generic requirements GR No 24 I would have to re read it to see if it covers private as well as commercial operated aircraft, as the case used to be just commercial IE public transport.
Continentals TBO data is here
http://www.tcmlink.com/pdf2/SIL98-9A.pdf
what country are u in? again ask your engineer about ohaul periods, he should know