Originally Posted by
taylorgeo
1. Frame failing at the head tube
2. Front wheel coming off open fork dropouts
3. Cracking the seatpost
#1 I believe should not be much of a concern. It is growing more common for mountain-bikes to have 'tapered' headtubes, narrowing from 1 1/8" up top to 1 1/2" below. A much rarer standard (usually only tandems) is 1 1/2" top and bottom. But the standard 1 1/8" threadless design should be sufficient.
2. is not a significant risk either. Forks have had 'lawyer lips' for decades to prevent this. And since you've got this on your mind, you can protect even more against this by checking the front quick-release every ride -- a 5-sec safety check. OR you could get a nutted front wheel instead of a quick-release, OR more and more bikes nowadays have 'thru-axles', which also eliminates that concern (not as easy to find for under $500 though)
3. I would stay away from carbon, stick with aluminum, and make sure you can stick it into the frame a couple inches deeper than the marked minimum-insertion line, and you should be fine.