Check out
http://www.hasenwerk.ca/ This guy is a one man show in Quesnel, BC, about a 9 hr drive from Seattle. He does excellent work but charges an arm and a leg to cover his extensive R&D. I have a '83 Vanagon Westy and have been thinking about converting to TDI for quite some time. The main issue I have is that my wasserboxer is better then new due to major work on it 15,000 kms ago and the conversion to a TDI would be about $15,000 at least. For the amount of driving and camping in my Westy (Mint condition but only worth $10 grand) I cannot justify making the switch. I don't want the van to be my daily driver either.
The main issue you will have is that you have an air cooled van and would require extensive modding to turn it into a water cooled rig (rad and cutting to front end). Also, Vanagon Syncro's are easiest to convert because of ground clearance. Unless I encroach into my lower sleeping area, the ALH TDI would rest very low to the ground.
If you wanted to stick with a gasser, again you will need to convert your bus to water cooled. You could go with Subaru pancake engines or regular VW 2L 16v or 2.0 T engines.
Good luck!