Any Sandman is worth having, but condition and all original numbers is the key. A HJ V8 Sandman is a good thing, if it is in good condition (not super rusty) and the chassis rails match the ADR tag then they can be worth pretty decent $. Even better if most of the original Sandman stuff is still there and the original engine too. The only hassle with them is some people look at them with blinkers on, sometimes buy a car for too much and either give up when they realise what it is going to cost or over-capitalise and end up spending 50% more than what the finished product is actually worth. Or make the mistake of investing too much in a car that no longer has its original chassis rails. My personal take on it is you really shouldn't consider a nuts and bolts close to concours resto on anything short of at least a 308 manual as the return probably isn't there (not just $ but your countless hours). Nothing wrong with building up a nice van or ute out of a 6cyl or 253 Sandman but not spending the bulk $ required for total originality using lots of NOS stuff. Even stopping at 80-90% original (like making it look stock body and trim wise but underneath all rebuilt using what is available repro or secondhand but not correct for the car) can save you huge amounts on a resto.
Having said all that if you want to build up a nice modified ute or van and you can start with an original Sandman go for it, it'll have a higher intrinsic value than a 173 column shift drum brake car will! And if the car is run down and missing lots of stuff (making it relatively cheap to begin with) why not do it?
Also unusual to find a HJ 253 M20 with a 3.55 diff, normally they are 3.08.