For the OP, $350 wouldn't be a stretch at all. But to the other guy, I got the impression he doesn't currently have a mountain bike to use the components of. $350 for a full build seems like a stretch. Though you obviously have more experience than I do, so be aware that I'm just trying to get an understanding, not argue.
When it comes down to it, just do what sounds the most appealing to you. I personally like building a bike & learning more about bikes because I'm still new to the maintenance aspect of bikes in general. But the majority of the people out there would rather just get a bike & ride it, so if that floats your boat just search around for good deals & report back here for more advice.