The 3800 can last into the 350-400k mile range or more with proper maintnance.
The 4T65 on the other hand has an average lifespan of about 200k miles.
The upside of the powertrain on a 3800 is that replacement parts are easily available and cheap, especially if you install them yourself.
The downside is that your vehicle is 13 years old and has a value, which is probably just over double what the body work will cost you.
What you do is up to you, but I would save that $1,500 and get a clean bodied car later, and keep this one as a daily driver if thats something you're interested in.