Swap 2.5tdi into S3 88" ?

I have a '73 Series 3 2-1/4 petrol which is in good shape above and below. I bought it because it has a newish galvanized chassis. It's 99% stock.

At the very least, it needs an o/d and while we're at it, a little extra power wouldn't hurt either. Half again as much grunt as stock would be perfectly sufficient.

I wonder if anyone here could advise on the most cost-effective way to go. I don't want to change out the axles, etc. but I'm not necessarily averse to an engine swap if I can find someone in southern New England to do it. Obviously I realize that it's a no-brainer to upgrade the front brakes to discs while things are apart.

- is it brutally expensive to upgrade the whole thing to a 2.5tdi? My crude understanding is that it practically bolts right in. Who does this sort of work? Are we talking about gigantic wads of cash? I'd be very psyched to be able to run on biodiesel
- I assume the Roamerdrive is the best way to go as far as an o/d?

I realize that if I soup up the power at all, I ought to put disc brakes on at least the front. Which brand is the simplest to swap in and go?

Also, minor question: re door glass: the PO did some slightly wierd things like put in angled front door glass (I think they had that on some Defenders). Can I just replace the glass itself or do I have to order whole new door tops?