Just in case you're worried about the instruments, EA and Harmonix have a fairly liberal return policy for malfunctioning controllers. Do to some incomplete QA testing, a lot of the original issue instruments had some defects. I had no problems replacing a wonky Strat. There are people who bought this game in November that our still swapping out broken instruments, well past the 60 day warranty.
As for the incomplete set, RETURN, RETURN, RETURN. They shouldn't be reselling incomplete items like that. It's not a bundle unless all of the components are there.
In any event, (call me a fan boy if you wish), Rock Band was definitely worth every penny I've put towards it. If you can get it at a steal, go for it.