The bottom ends of waterboxers are pretty good.  Most will go 150,000
to 200,000 miles or more, easily.

Just do the heads for now.

Who knows, you may have to redo the heads in 20 to 30,000 more miles
if your mechanic is not a waterboxer expert.

If you want to keep you van forever, consider an investment in a
Subaru engine.  You'll have much more power and reliability, but it
will cost $7-$8k by the time it's installed.