Just be guided by the info on the internet on

dit.sa.gov.au/outbackroads

This gives a good run down on closures, conditions and warnings on the roads you are talking about. The road from Marree south is now fully sealed so wont prove an issue.

Your van should be fine if driven conservatively, and you have prepared the underside to protect pipe work, gas lines etc.

The cruiser will be fine, that is what we have used for all our track trips in that area. We are also from Adelaide.