I think your plan to drive the bug for a couple years is a good one. I also strongly recommend that you avoid buying an Escort. The reasons are many, but I'm getting tired of typing today, so just trust me on that one.
Another alternative that I think is a good idea would be to buy a cheap 4 cylinder Mustang, don't put
ANY unnecessary money into it, and drive it until you either save enough money to buy what you really want, or to build a new V8 to transplant into it. I personally would recommend saving up for a 2nd Mustang, rather than transplanting one. My logic is this: if it's a Mustang that you want to eventually have as your musclecar, starting with a 4 cylinder Mustang would get you used to it. You would become familiar with it, and would transition to a V8 Mustang much more smoothly. You would become familiar with ways and means of obtaining Mustang parts, and would be better informed as to the options and accessories you would want. You also might decide you don't like the Mustang afterall, and wouldn't have dropped a small fortune to discover this. Plus, if you do end up with a fast Mustang later, you will still have a respectable car for chasing down parts, or for times when you would rather get someplace cheaply instead of quickly.
The VW is a great idea, too, but being new to the automotive world, keep in mind that whatever you start out driving will influence your future decisions. After 2 years of driving a bug, you will probably have read dozens of VW magazines, and met new VW friends, and may decide that building a fast bug is what you really want. Going from a bug to a V8 Mustang can have dangerous results, too, as far as driving habits are concerned.
Besides, if you buy a 4 cylinder Mustang first, you can still enjoy all the Mustang t-shirts and books you will end up buying before you turn 18, after you turn 18. Get it?
Take care,
-Chris