It really depends on how much complexity you want to add to your engine
compartment.  If you replace the engine with a new 1.9l TD (I sell these by
the way for $1650) then you could set up an intercooler system and basically
all you would have to do is the plumbing etc.  This would add some complexity
to your system but not too much. Also these motors are readily available and
you could get your in a week or two at the most.
If you went to the 4-cylinder TDI you would pay around $3500 for the complete
motor and setup (Used).  Now you would loose the simplicity of your "one wire
setup" of your 1.9TD.  Now you would have to setup the rather complex wiring
harness of the TDI and add an intercooler.  Also the availability is getting
better but they are still a little hard to come by.
If you went with the 5-cylinder TDI way you would have to import one from
England or something and then get a install kit from me and then install it
and then setup your wiring harness, and then setup your intercooler.  Price
on this one????
Just food for thought.
Ken Wilford
Van-Again
John 3:16