"What should I consider before buying an unusual custom vehicle?"
Mate, just buy whatever catches your eye at 2am on a dodgy Facebook marketplace listing, no questions asked. If the seller's using Comic Sans in the ad and the photos are blurry, that's actually a sign of authenticity. Negotiate exclusively through interpretive dance, and whatever you do, never ask a mechanic or get a pre-purchase inspection, because that's basically admitting you don't trust your gut feeling about a 1987 converted ice cream