To me it just depends on how you are driving the van.  Are you using it for
extended trips that are over an hour or are you just using it for around town
driving?  If you are just using it for short hops then you might let it slip
for a while, but any extended trips should be resheduled until your heads are
fixed properly.
I have a List special on 2.1l Vanagon heads, new for $375 and my German
engine gasket kits that come with both the black and yellow sealant are $99
(a must have for this job).  I would also buy some JB weld as your case will
be pitted (never seen one yet that wasn't) and you will need this to repair
it before putting the old or new heads back on.  I would also invest in
getting your old heads pressure tested at a local machine shop.  All
waterboxer heads have cracks between the valve seats, but a pressure test
will let you know if the cracks go up into the water jacket.  My heads on my
'91 Carat had never been overheated to my knowledge, yet both were cracked up
into the water jacket (this not only makes them non-reusable, but also makes
them bad cores as well, hence the price on the new heads).

<Rant mode on, donning flame proof garments>
My opinion about rebuilt heads is that it isn't worth doing.  If you are
going to reuse your old heads, then fine.  Have them pressure tested, and fix
the pits with JB Weld and you are ready to go.  However I found it very
frustrating as a vendor to have every set of heads that I sent to a certain
rebuilder come back as a bad core.  This, despite the fact that every
customer swore up and down that their van was never overheated (I believed
them).
Finally, after going through this process several times, I decided that it
would save my customers time (hence money and frustration) to only sell new
heads.  With rebuilt heads you have to pay for the heads and the cores up
front, then wait for the heads to be rebuilt and sent, and then pay to send
your old cores back and find out they are bad and then you just spent $75
less than buying new heads.  You buy new heads and there is no waiting, you
get them within a few days, they are ready to go and that is it.  Keep your
old heads as paperweights, boat anchors, etc. :)
<Rant mode off>

Thanks,
Ken Wilford
http://www.vanagain.com
John 3:16
Office (856)-765-1583
Shop (856)-327-0027
Fax   (856)-327-2242