Before you invest more effort, time, and money,
The best thing about all this is that you pretty much have a blank canvas.. You can do whatever you want at this point. And depending on what you have stockpiled, or what is available and what you are comfortable with will ultimately drive your decision, but be mindful that some choices have already been made for you... ie: it may be the cheapest and easiest to buy another engine, crack it open, and upgrade things before they break, and literally start with a fresh motor with 0 miles.
Swapping to TDI is nice, but totally not worth it. The effort, and replacement cost trump every reward. Ultimately, you are better of selling it, and buying one already converted.
I faced the same dilemma when I decided to replace the engine in my Vanagon. I have already invested so much time, effort and money, that my choice has already been made for me -- another aircooled Type4, even-though a watercooled Subaru would have been feasible, but that would've meant I have to part with a $500 exhaust, and other new items I already had for the Type4.
Learn to ride. Ride to learn.