There are a few other considerations worthy of note.  How many miles do you
drive your petrol vanagon per year?

Diesel here in Governator Ahnold's hometown (Sacramento, CA) hovers around
3.60/gallon with petrol at 3.15/gallon. So you pay about 0.50 US more per
gallon for diesel.

If you drive 10000 miles a year and get say 20 mpg with yer gasser that's
roughly 500 gallons 3.15/gallon so dividing 3.15 by half to get 1.58 and
rearanging various decimal places you spend 1580 USD per 10000 miles at
today's optimistic prices for gasoline.  If you get 30mpg (very optimistic)
with a TD or TDI on diesel then you buy 333 gallons of diesel for that 10000
miles.  With suitable dividing of 3.60 by 3 to get 1.2 you spend 1200USD for
the same 10000 miles with your TD/I conversion.  Per 10000 miles you save
380 dollars over petrol.

How many incidences of saving 380 dollars will you need to pay for lets say
a 10000USD conversion.  Just for an outside guess up the savings to
400/10kmi.  Jiggling the RCH just so and slippin and slidin shows that a
mere 25 instances of that savings will pay for that 10000USD conversion.
That works out to 25x10K mi or 250Kmi.  You gonna get that many more milas
out of your 20 year old 150Kmi vanagon?  Perhaps.  Then it would have
400Kmi.

Set up the model in your spreadsheet and tiddle the figgahs until you have
your favorite alternate reality.  Then toss in another 2500US to upgrade
wheels, tires, and transmission (conservative estimate).

If it makes you feel green to spend that kind of green, don't let the math
dissuade you.

It's still frydaye here in the not so golden state (14B$ projected deficit).