Hi Mac, and welcome.
There are quite a few older PTO generator units around. You might be able to find them at auctions. I have seen them as units on trailers, as stationary plants you back the tractor up to an connect a shaft and as units that mount on a three point hitch.
I have seen therm in almost every size, from small units to power a few lights and skill saws to units meant for emergency power to hog barns.
They are still being made new too, but are not a common item in the stores.

Here is a small unit somebody put on a garden tractor.
http://www.mytractorforum.com/showthread.php?t=64583

Here is a fact sheet on PTO units from the Ontario ministry of Agriculture.
http://www.omafra.gov.on.ca/english/engineer/facts/00-059.htm

Here is a catalog page from a manufacturer.
http://www.wikco.com/generator.html

As you see, they are not cheap units to buy new and the fact that you need to be have a tractor available to run them makes them less attractive than units with their own motors for most people. (It means they command a lower price when sold used)
This means that tractor powered units sold at farm auctions are usually in pretty good condition, at least as long as they have received any reasonable care at all.

If you decide to build your own you need to think about how to get the right number of RPM on your PTO. You might be able to use a gear transmission or belt pulley transmission.

Remember the generator set has to be secured to the hitch or the ground well enough that the PTO does not simply start flipping the whole thing over. It takes quite a bit of torque to generate power.

edit:
Variable power should be about the same as most other gensets with the tractor's governor controlling the motor rpm and the field strengths in the generator controlled by the load.