Welcome to the group and congrats on considering the purchase of a Sunline.
Sunline is now out of business, but don't let that scare you. They were a very well built TT compared to a LOT of what else is out there. It was a sad day when they shut down.
As for the awning, cost to repair/replace will depend on "what" is wrong with it. If fabric is ripped you can purchase replacement fabric, if any of the mechanism is bad, I believe you can also find just about any part you'd need to repair it.
Can't tell you too much about the trailer specifically without knowing the model #. That should be on a plate outside the main entrance door, or on a spec sheet located inside one of the cabinet doors.
If you can get the VIN #, send it to SunlineFan in a privae message and he might be able to tell you more about the Sunline in question. He is our resident Sunline historian.
Also, if it would be possible for you to get pictures of interior & exterior and post them, there are many "eyes" on this site that could give it a look over to see if anything unusual jumps out as a problem.
Sunny #18 2003 F-344SR #8157
Sunny #19 1997 T-2653 #5485
1999 Ford F350XLT DRW Crew Cab, Long bed 7.3 PSD
2002 VW Jetta TDI.. AKA: Kitty's Kruiser