$400? that's how much mine was, and it was a rolling wreck when i got it, though in our market it was called the GSX-250E instead. GS-250T, GS-250TT, GS-250TX, GSX-250E are all basically different people taking turns at doing the styling on the tank, seat, side covers and dash.
I can tell you that a lot of people buy then, run them into the ground and then sell them off to some other person willing to throw the time/money into the project of getting them working again.
You've encountered a common problem already, mufflers. Is it a muffler leak alone? or is there a problem with the headers too? If you have a 2 into 2 one like this (
link), they seem prone to rusting out quite badly and it would be in your best interests to try and find a 2-1 header as these are lighter and a lot easier to put on/take off as needed.
Carbies definitely need to be set up correctly, as some (including mine) were outrageously neglected and choked up, and it was a wonder it was still able to start, let alone run when i first got it. Stripping the carbs (as per the imminent BassCliff welcome wagon/guide) and cleaning out all the nooks and crannies thoroughly is a good first step. Then sync the carbs and plug her back together.
I would also suggest consider getting a compression test done to get a rough idea of the condition of the cylinders, but given its only got 12k (miles? km?) unless the past owner was brutally neglectful, it should likely be perfectly fine. It may still be handy as it'll show if there is any leaks past the valves or not when done correctly.
Another thing to do is check the valve clearances, this is an absolute must as these bikes can have their clearances wander a little, especially as they have to work so hard to get anywhere.
As to NOS, available, but expect to pay through the nose if buying genuine off a shop, ebay would be your best bet, or alternatively look into getting a single pipe and buy an aftermarket one if as mentioned above your 2-1-2 (if that's what you've got) is shot.
If you want to have a look at some component parts of a GS/GSX then feel free to check my
flickr photoset out, though hopefully most of these won't be relevant to you as i got up to my elbows deep in engine a couple of times in there.