Oh yeah how do we love it that every state has its own rules for conversions and they have not to accept the mod of another state. Combined with a 30+ year old conversion, things are getting even more messed up
I got my Syncro from NSW to VIC, fully fitted Camperize conversion with pop-top pretty much from day one arriving in the country. There is no mod plate fitted, only a sticker from Camperize with an approval number on the inside of the A-Pillar. What the meaning is behind the number is a secret to me as Camperize is long gone (and I haven't bothered to investigate with Service NSW if there are some records to that number).
I guess I was lucky when I went for an RWC with the local mechanic as he didn't say anything about the modification. Also I haven't had to go to VIC Roads for in inspection when I registered the van (which I guess was lucky). Paperwork still says Window Van, 2 seater (though it has got 4 seats)
Long story short, nobody can tell you what will happening when you buy a car of that age interstate with any modifications. You may just slip through the system, or you may get hit up with a hefty bill to get it locally approved. PITA...
As for Insurance I'm with CIL - they are price wise reasonable on an agreed value and specialised for campers. Only needed to send them pictures of the van to prove the campervan conversion.
Cheers
Arne