Yeah, I'm not sure if I'd recommend an Ultra1/2, they're a tad slow, and as Nothinman said, SBus based, which isn't bad in itself, but finding SBus peripherals is rather expensive.
The Ultra5 and 10 are pretty much the same thing, except the 5 is a desktop and the 10 is a tower.
Oh and the 5's case sucks bigtime, AST made them back in the day IIRC.
And to answer the original question, yes I think it would be worth it to get a SPARC box if you're gonna learn Solaris, some things are different, and you'll get to play with Open Boot, which is nice to know.
I'd also try to get my hands on some course material, shouldn't be that hard to come by, and the materials for their Solaris Administration courses, don't know what they're called these days, when I took them they were called SA-238 and SA-288.