1) MeeGo is not ready for daily use, probably not until April, and as it will be a community supported project may never be acceptable for daily use depending on what your needs are.

Android (nitdroid) is close, but does not yet have voice calling support or GPS, and has a few other issues. Hopefully these issues will be worked out sooner rather than later.

Ubuntu, doesn't have voice support etc, probably usable as a mini computer, also check out easy-debian.

2) See above, I wouldn't consider it usable at this point.

3) USB Host, not possible at this point. There has been some recent successes, but nothing really ready beyond very alpha type stuff. Hopefully, at some point this will be ready for end-users.

4)Overclocking can certainly damage the N900. Extent and probability are unknown. If you are concerned I wouldn't overclock. I don't think there have been any reports of anyone killing their N900 from overclocking yet.

5) luck of the draw. Some units do better than other units.

I would suggest you purchase the N900 for what it can do today, not what it may be able to do tomorrow. Otherwise, you may end up regretting your decision when some of the things you are looking for don't turn out like you expected / hoped. Also bear in mind that the N900 is not a phone and is missing many basic phone features that you may be used to (group ringtones, voice dialing, etc), some have been addressed by the community, others haven't.

I would suggest that if you are asking these questions, you haven't done enough research, and probably shouldn't buy it (at least until you do a lot more research).