Bafang seems to be what people install, plenty of examples in this thread.
Agree that 500 is very unlikely to get you an e bike that works.
Is swapfiets in London? It's a long term rental thing and their ebikes is what almost all delivery riders use in Berlin.
Don't think you'll have much luck finding and ebike for that price second hand, there'll be some ropey DIY deliveroo bikes on there for that mark.
Would be better off putting the £500 to converting yourself. For £600 you can do a decent conversion.