TL, those are all very achieveable goals.
My first engine choice was a Type-4, but being a College student with a modest budget. I can't very well sit around 2 or 3 years while I save cash for the engine. I decided to go with a Subaru because of the out of box power you get. They were built to go 100,000+ miles with 200+ hp. I ended up buying mine for about $1,000 JDM. You can't touch a big T-1 or a big T-4 for a $1,000.
Your transmission criteria makes perfect sense. I'd like the same type of transmission myself. I'm using a 914 right now. But I'm not sure how well 1st will hold up.
I want the best for the money, and I'd rather do it right the first time then have to redo it later.
I believe, it is a shared opinion among us water converters to use a much stronger engine to start with. If you like being under the hood of your bug often. Then a big T-1 or a big T-4 might be good. But if you want to hop in it and drive, with an unbelieveable amount of HP and Tourqe. Subaru.
As for later tuning. Ummm... if 200hp in a bug is crazy. Then 300hp in a bug is insane! To help keep cost down, you can always leave it stock for now. Then tune later, also if you think you want to tune it later. Go with an aftermarket ecu over the stock unit. They're about the same cost, but the aftermarket is tuneable later.
I read somewhere. Power, Reliability, and Cost. Now pick two. I'm trying to find the best of all three.