I own a 97 c2500 with the 6.5, and I have over 200k on it.  If your going to buy the truck the first thing you need is to look for a PMD Heat Sync. It should be mounted in plain sight on the intake.  If it has it the first big problem is taken care of.  After that if the truck is going to haul that much weight(which it can) You ahve to upgrade the cooling system.  SSDIESELSUPPLY.COM sells all the kits and parts you will ever need for your engine. But I'll tell you it will cost about 500 dollars. and that will include a new High Output water pump( you will have to have it) but it cost the same as a normal waterpump, High Output 180 Degree T-Stats, and the Duramax Fan upgrade.  

Here's my 2 cents. If I were you I would keep looking around. Maybe even look on eBay, there are usually some good deals on there! Bottom line is that 6.5L Detroit is a gutless pile. It should get good mpg but other than that it is extremely gutless! We have a few military surplus 6.5L & 6.2L chevy diesels over at the ranger station where I work at and they do get good mpg but good lord are they powerless! If you are looking at towing on the heavy side then forget about getting a 6.5L. Seriously you will regret getting it, especially with that many miles on it. The interior looks real nice as does the exterior but that motor is absolute crap. I understand you don't have a lot of cash & that's ok because chances are you might be able to get ahold of a 7.3L Powerstroke. Seriously look for a 7.3L powerstroke or Cummins because those are the routes you want to take!