your looking into a bike that doesn't really work well. you said you are going to race cross, xc or whatever, but really a mob would suck for that. Aluminum hardtails work for MOUNTAIN BIKES, but are usually a little tall for a good street whip. The mob is a great street bike but isn't that awesome for riding off road, you can do it, but the angles are off. the steel head is designed around running a dual crown fork if needed, so the head angle will be pretty slack for a street bike too. a real street bike is tiny, and should have a 12-14 inch seat tube max. look for chainstays around15-16 inches at the most, and never run a fork bigger than 100. that being said, if you want a freeride hardtail, which is less of a bmx bike, you should be able to run it geared or not, and have it a lil taller. it won't be that dialed for street though, but you can run a longer fork.
i can hook you up with a basically new brodie holeshot frame, sized small 14'', painted custom copper, and a marzochi 4x fork for about 600 if you want, that's about as utility hardtail as you're going to find. lemme know.