Just to ad to Chucaro's post (and to avoid a war :D), it's not unlikely for a well-maintained 300tdi to go for a very long time, and for a poorly-maintained one to require head gasket, etc.
If you are looking at a poorly maintained example, I would start budgeting at $2k for work, and go up to $5k, if as likely you will need a whole bunch of cooling system work, possibly a new head, etc.
Simon

