I'm fairly certain most generators produce an AC output with a proper sine wave. It's a rotating piece of machinery and can't help but produce a pure sine wave. The only caveat here is, if it's a DC generator and uses an inverter to produce the AC. It's the commonly available solid state inverters that output a modified or chopped AC signal and the pure sine wave solid state inverters are big $$$$. Certain equipment as you've been informed, does not do well with this chopped wave, generally motors and such.
Before the days of solid state inverters, we had mechanical rotary inverters also referred to as motor-generators. As implied by the alternate nomenclature, the DC power supplied to the unit ran a motor, directly coupled with a generator to produce the AC power from the DC source. The resultant output is a nice pure sine wave. These things were maintenance intensive and noisy though. Both motor and generator had brushes to wear out, not to mention bearings and all the other moving parts.
With respect to generator quality and price, that can be quite a challenge. The more they cost, generally the more power and safety features they have. The build quality will be better as well. There are so many variables, including the engine or whatever is used to drive the generator. As I mentioned, the sine wave output should not be an issue if it's a proper AC generator, not a DC generator with an inverter. Don't think the latter is all that common. What can affect the output is a surging motor, but if the unit has a quality regulator, frequency and voltage drift should be minimized. So if you have a little cheapo unit with a two stroke screamer driving the generator, you may want to keep an eye on things. If you've got the $$$, look for a higher quality unit for peace of mind. Not much you can 'add' on to most generators to improve the output. It should be designed to output the stated voltage & frequency.
