Welcome to Sunline Club, BillsFan!
One thing you may find helpful is to go the the Files section of this website, select Sunline Brochures, and then download the 2003 Sunline Travel Trailer Brochure and print it out.
In that brochure, there are 3 Slide Room models (the smallest of the bunch) that have a GVWR of 8,600 pounds. The remainder of the SR trailers (including the T-299 which I think may be the one you are looking at) are all 10,000 pound GVWR trailers.
For the 8,600# units, you would be about at the safe maximum towing capacity of full size SUV's like the Suburban or 1/2 ton pickup trucks (even the "heavy halfs" that were available for a while.) Most of us would suggest that a 3/4 or 1 ton pickup are much better tow vehicles at this weight range.
At 10,000# GVWR, you're pretty much limited to 3/4 or 1 ton pickups for a tow vehicle. Half ton trucks don't cut it at this weight, even though a couple of manufacturers spec the latest models to tow this much. IMHO pulling a trailer in that weight range with a half ton pickup is unsafe.
Now on the other hand, if you are going to park this thing on a seasonal site and never drag it anywhere, forget what I have written about towing....
Like Kitty said, double-double check for any signs of water leakage not only from the roof, but check the floors for soft spots that might indicate water damage that is not visible.
Make sure that there are no leaks in any of the plumbing systems. For example, pull drawers under sinks (there's a little tab you bend forward at the back of the drawer and it'll slide right out) and look for any sign of leaks.
The refrigerator is supposed to run on 12vdc/propane and on 110vac. Make sure it works on both. Check to see if the recent Dometic recall has been performed on the unit. If not, don't worry about it now; you can call any RV dealer/shop who services Dometic and they can do the upgrade. Elsewhere on this site there is a sticky about the recall and how to proceed It's easy and once the shop sets an appointment, it should only take about 30-45 minutes for them to do the job.
Check all the lights. Look for signs of abuse or damage, inside and out. Inspect the tires and see if there is excessive uneven tire wear. There's a great post in Repairs/Maintenance here about tire wear issues.
Make sure that the unit has been properly winterized and that there is pink RV antifreeze in all the sink and shower traps and a bit in the toilet bowl. If possible, turn on the pump and open a faucet briefly to see if pink stuff comes out. Winterizing and de-winterizing is no big deal. Almost all of us have to do it, even the folks in northern Georgia. You'll find several posts about that here, too.
Pull the plug on the hot water heater from the outside service panel and make sure it was drained for the winter. Obviously you can't test it if it is properly drained, but you can inspect it.
Just check everything as best you can. Check the microwave, stereo, clock, smoke detector, propane detector, etc. etc. etc.
One thing I would point out is that a 6 year old Sunline in good shape is a really good thing. Even though Sunline is no longer in business, all the installed appliances and components in the units can be serviced by almost any decent RV shop. The magic of the Sunlines is in the design, materials, and assembly.
As soon as you can determine the exact model and let us know what is in it, particularly airconditioning, Jon can look up a blue book price for you.