First off, Is it a good fit for the style of camping that you will do? This is important because after you devote years of unrecoverable time and wheelbarrow loads of cash, you don't want to find it to be too small.
"QUARTET" sounds like four people.
The GT was designed to accommodate four, but that's just a marketing ploy. If you toss the side gaucho and build a dinette in it's place, they're perfect for two. The original dining configuration is just plain stupid.
67/68 GTs are really cool, because they're considered a small Airstream, and had one of the best ever bath full configurations. At less than 3000# they can be towed by nearly anything that weighs 4000# and 200hp. Mine tows beautifully behind a Dodge caravan.
$2500 is a fair price for what you have to consider to be a "rebuildable core". I paid $5,500 for my rebuildable core, and I'd do it again. In fact, I love mine so much that I'd consider buying the one you're looking at if you don't want it... Last year of the narrow body and wood interior. They're great "campers".
Unless it is a remarkably unusual deal you can plan on having another $10,000 into it as it approaches its "finished" condition. Truthfully, they are never finished, even if you buy a new one from the factory.
No matter how many times you scour the interior with anti-stink products, on warm days it will always have "Eau du Ro"(dent) smell unless you remove the interior skins and freshen the insulation. Expect a 30gal shop-vac full of acorns, walnuts, hickory nuts, pine cones, etc. between the floor and bellypan, maybe a raccoon skeleton or two.
Figure shell-off restoration with newly fabricated frame as your worst case scenario.. Get a good look at the frame before you buy or assume it is bad.
If you love tools, are a patient and good natured individual who can find enjoyment in the restoration process, definitely go for it.
Like many others, there are few things that I'd rather be doing than fixing up my trailer. If that doesn't sound like you, move on to something more manageable...
In another recent post, someone asks "Where is an Airstream's soul located??"... That's easy, IT'S IN YOUR CHECKBOOK.
There is a faction of Airstream restoration purists, and that is a much needed and commendable avenue, but most of us want to make them "Our Own", so make it "Yours", and you won't worry much about finding all those old odd parts.
Link to my GT...
P.S. Please regard these comments prefaced with a polite "in my opinion"