Looking at a 300D
 

So, I've decided to try out the world of the Mercedes Diesel. There's one for sale locally that runs, but it has transmission issues. It's a 1983 Mercedes 300D. I drove it yesterday, and it won't shift out of first gear, and it slips the entire time you're driving forward. It was to the point that it would just rev up instead of moving up a hill.

However, in reverse, it drives just fine. It will climb hills and it doesn't slip at all. My first thought is that the transmission needs an overhaul (which is a deal-breaker for me), but I've been told by a local diesel mechanic that there's an off chance it's a vacuum problem.

This car sat for 2 years and has 291k miles on it. It also leaks transmission fluid (from the pan, I think). It has also been converted to a 2-tank GreaseCar kit with a switch between diesel and WVO. The body isn't in the greatest shape (kinda rusty), but I'm just looking for a highway car.

Now, I'm not new to diesels (I've had a lot of Cummins and IDI trucks), but this is the first time I've ever messed around with an automatic that's vacuum controlled. How can I tell if the transmission just has vacuum issues? What should I look for in a car like this otherwise? Is it worth the roughly $1000 he wants for it?