used power amp under 1500

I started with a great system and I managed to downgrade it to an OK sound through a lot of trades, sales, questionable decisions and shrinking budget. (No regrets, it's been a fun journey.) I ended up with very low sensitivity, hard to drive speakers and now I need more power. Will 100 Watts be enough and is it OK and safe to buy used? Any brands that are a good value at $1500?