Shouldn't the venue be responsible for adequate power for the event? Why should this be YOUR problem?
BTW, you are not considering BUYING one, are you? So what's available for renting in your area is probably the main thing...
If it were me, I would tell the venue how many amps (electricity) roughly you need (how big is the PA and lights?), and then I would run anything like keyboards, effects, mixers (anything that doesn't consume a LOT of power, unlike power amps and lights) on a UPS battery backup system (plugged into the generator), so a brownout or surge doesn't damage your gear...
If you rent a system, make sure you also rent maybe 100 foot of heavy duty power cable, too, so you can get the generator WELL away from the performance area