Just a quick look on Craigslist in Austin Texas came up with a 2000 Ford Excursion V10 for 5950. I have a 2001 Ford Excursion with the V10, and I can tell you I test drove a few before I settled on the one I have now.
There's also a 2003 Ford Excursion V10 with 130k miles for 7500. Point being I think there are a lot more options out there than just a 1500. If I was towing for six months I would want my older rig based on 3/4-ton parts. If you can find a 2500 Suburban then spring on that, just keep your eyes open I think you will find a solid vehicle.
As for reliability the Ford V10 is known for lasting longer than the body it's put on as long as it is maintained. The GM 6.0 is also another one that is a great long running motor. The bigger thing would be finding the 3/4-ton chassis of either the 2500 Suburban or Excursion for the additional payload, breaking and handling.
