Where I live we aren't allowed amps anymore. We used to be but it turned into amp wars. Alanabit is right on the money. Buskers are trying to make money and that pressure will cause many to ignore ettiquette and common sense/respect. I watched it happen here. In fact, it still happens, as we have marimba bands with full kit drummers drowning out everyone for blocks sometimes. The funny thing is they look like they would be "hippies" or "tree huggers" - long hair, tie died clothes etc - and would follow those sort of ethics - love, respect for others etc. Very much a generalization, I realize. We call them "loggers" because they clearcut the rest of us.
Amps - I used to use a Peavey Solo - compact - decent sound - not too loud (!) - 8 "D" cell batteries - one set would last the summer - two inputs including xlr for vocal mic.
Singing acoustically - I used to get very hoarse singing by a loud corner. I play over 4 hours daily, sometimes 8. Now I have found myself a quieter location and my voice holds up just fine. People will be drawn in to listen to quieter acts as long as the location isn't too noisy.