We bought a high mileage truck to keep the initial investment down. 5 years old with 836,000 miles for $27,000. With a truck like that a good inspection is a must which we had done by a Volvo dealer. They also did a dyno to check the quality of the engine and the tranny. About $800 total with an understanding that if it didn't pass I was out $800. Dyno passed but the truck needed about $1,500 of various fixes which the seller agreed to have done by the Volvo dealer.
We had the truck for 8 years and it now has close to 900,000 miles. Over those years we had around $15,000 of repairs, Three big chunks $5,000, $2,000 and $2,000 which we had done on the road and others which we had done in a more "leisurely" way while sitting at home or in a campground. I don't mind fixing things, or having it fixed, it's part of life. If that freaks you out, an older HDT is not for you. Repair on these things are more expensive that on a car or a pickup, because the repairs are expected to outlast a pickup or two. You are playing with "big toys" now. But all in all I am still way below the price of a new pickup puller and I have a "real truck". The reason I wanted a "cheap" HDT because my plan was to outfit it with a fancy deck and drom which was going to put the whole thing between $50K-$60K. So far I'm "on the money".