The beauty of having a standard van and not a genuine Sandman is you can build it up to whatever you like... ie doing up a HJ as a HZ Sandman... You can put GTS gaurds on it, a woodgrain dash if you like and all sorts of other goodies and the "purists" won't pick the eyeballs out of it..

Why do you want to put a HZ chassis under it? What is wrong with the HJ chassis?

The chassis number dictates model and what ADR's it complies with.... your State rego dept. will have these on file...
EG..
CHQ00345S

C= third ADR change in the HQ model run.
HQ= obvious I think....

$900 is a good buy for a windowless van and the beauty about restoring it yourself is you can gradually restore it as you have the funds available rather than one big initial outlay..... plus it is fun to do, you learn heaps about your car and you can look back and be proud of what you have achieved.....