Do be careful not to buy something that has been abused to the point it is ready to fail. Specifically smell around the motor for burned smells (showing it had a major overload long enough to overheat the motor) and for other wear.
Easiest is the specification plate that *should* be found on each unit. Figure *roughly* that 10 amps is about 1 HP for an electric motor. (the number is less, but inefficiency in the motor and system drive it higher). So ignore a '2.3 HP rated' sticker, and look at the power drawn on the nameplate for a good indication of how powerful it is.
That said, power is not critical unless you need speed. You can cut a lot of wood with a less powerful saw, it just takes (much) longer!
Absolutely! A good high quality blade is really important. I've used saws with bad blades (and bought at least one for virtually nothing because the blade so was dull that it would not cut, and the seller didn't realize it was just a bad blade, and not the saw).