Not sure what searches there are in Victoria. But make sure you get it searched both chassis number and engine number wise to make sure it has no theft or other encumbrance on it.
You can also try an AOMC search, these were by engine number but if it’s been regoed in VIC it should come up. It’ll be easy if he has paperwork showing it’s 186S engine number.
As for the engine swap, there isn’t much needed to turn a standard HT GTS into a standard HT V8 GTS. The brakes are the same. The rear springs are the same except the GTS has a large front spring bush (V8 GTS is the same as HK GTS). I think the front springs are the same. The main difference is the larger uni joints on the V8 GTS meaning V8 banjo diff centre and tailshaft with larger front and rear unis. What I mean is even if there are minor hoops to jump through rego wise, as in either self engineered or by a consultant Engineer, it won’t be difficult. Where it might get hard is if he butchered the main floor crossmember to fit something alien like an Aussie 4spd or Toploader or similar. That floor member is a large part what supports the front seats.