George...it's easy to use. I've used it many times , on and off the engine. Just make sure you get as much of the valve spring as possible under the claws....easier to "bang" the keepers loose and remove them that way.
And yes, use compressed air in the cylinder to hold valves up while you have the spring(s) off, or, like 4spdragger, rope stuffed into the cylinder works too. If you don't do one of those, you may end up with the valve in the cylinder, and that never ends well!
As for seals, I'd just go with std. umbrella ones...like you say, the old only went 209K miles and 23 years!!...LOL!!