Most of the time they raise their voice as a diversion to intimidate and avoid answering the question you bring up. So when people get angry, there's really only 2 situation - they're truly triggered because they're the real thing, or they're total fakes.

If they can't speak proper English, do they seem like a local or from a different nationality? ---> asking this out of curiosity lol

Perhaps inform the guard to blacklist the said car plate and person. Next time he tries to enter, tell the guard to deny him entry.

Banks and legit financial institution doesn't randomly start sending such notice or debt collector if your payment record have been consistent. You were right in questioning him and repeatedly trying to verify information. If it's me, I would have demanded for his work identification and also take his photo or try to video the incident saying I'll have to verify his identity with the bank (if he's truly their rep).

In future it'll also be good to install CCTV at the entrance in case he ever comes back, or a different person but similar tactic appear. Tell them this entire interaction is being recorded. Honestly it's lucky that you were home. Imagine you're not in and both your elderly parents have to deal with him. It's likely they will get intimidated and ended up following what he had to say.