Hobby machines are hobby machines, you are talking about stuff thats old but not old enough to be completely obsolete and that is different. G5's will get cheaper, a working SE/30 will only get more expensive.

Its your money, only you can know what you want to play with. The only thing people here can warn you about is overpaying, or system issues to look out for. And if you think Apple is going to cover any warrenty work on a G5 system these days...... good luck with that.