As far as mechanical stuff, I think I can tell you what to look for. I didn't really know what to look for when I got my Bug. An engine rebuild later, I know exactly what I should have been looking for!
Pop open the rear deck lid. Try to do a little studying on what the engine should look like "intact". Look to see if all the fresh air hoses, cooling tins and air cleaner are there. I'd bet you'll find a little round aftermarket doohickey for the air cleaner. That's fine for Summer, but in the Winter, that stock air cleaner with the air hose that leads to the underside of the cylinders is going to be key for keeping your bug running properly in cold weather. I still have to get a stock air cleaner in my Bug before Winter comes this year.
Another crucial element is the thermostat. Just like a water cooled engine, without the thermostat, the engine will never heat up enough (at any time of year!) and the car won't run like it should and the engine will not last as long. These engines can't run too hot or too cold. The thermostat regulates the temperature by opening and closing little doors that direct air flow. To locate it, stand behind the car, drop to your knees and look under and to the right of the engine. If it's a stock thermostat, it will look like a little accordian. If it's a new style thermostat, it will look like a big bolt with a fat spring rapped around it. If there it isn't there, and there's a good chance of that, plan on finding one and also plan on calling a vw junk yard to get the air flaps that it controls. Typically, if the thermostat is missing, so are the air flaps that is controls, which go inside the shrowd (that big half circle of tin that sits in behind the engine).
Also, if the thermostat is missing, it's also a fair bet that the car has been running a long time without it, which means the engine has a lot of extra wear in it, due to the fact that it has never heated up properly. When the engine doesn't heat up enough, it can run richer, which means less feul mileage. Further, there are no seals between the heads and cylinders. The heads seal themselves when they expand, due to the head, and they pull themselves against the cylinders as they expand. If the engine doesn't heat up enough, the heads won't expand enough, so that seal never fully seals and there is less compression from less of a seal and a richer feul mixture. If the feul mixture is too rich it can wash the oil off the wall of the cylinder if not fully burned and eventually the extra feul can work it's way into the bottom of the engine and mix with the oil. That breaks down the oil and it because less of a lubricant to the escential parts in the engine and the engine can wear out a lot sooner.
Wear dirty "work" clothes so you can get underneath. See if the bottom of the engine is soaked in oil. Run it to see if the engine runs without struggling or making more noise than a cat trapped in a bag of tin cans. If you are looking at a 1600cc engine (ask the seller), then test driving it shouldn't be much different than any other little car. It won't blow your socks off, but it shouldn't be a dog either. If it has trouble getting out of it's own way, it's a fair bet the compression is low, which will likely mean that you will at least need new pistons and cylinders. In my case, I rebuilt the entire engine. Everything was shot!
The good news is this: these suckers are easy to work on. Even easier if you've got your own paved driveway, and even easier still if you've got a garage! I had to uninstall and install my engine on grass and/or on my stoney driveway. Ooofff!!
I went from zero knowledge of how these cars work to rebuilding my own engine in a few months time. It probably would have been quicker, but I knew I wasn't going to have the money to put it to use, so I took my time in learning.