It has SMS feature, of course, it's a phone.
He means playing around with linux and if you don't mind that you don't have all apps your friends (might) have then go for the N900. It's only you must be sure you really want it. Because all "phone" features are pretty basic, for example no MMS support ( there is an application for that though)

2: N900 has a weak USB port so be careful (mine is still intact after 2,5 years, though) and screen scratches easily so a screenprotector is recommended too

3. Well if you have problems with the phone (aside that those problems are almost always the user), dont be afraid to google it up and read what can be done about it. Or some question about how to flash it, most of it's covered up in the wiki of maemo.org, so make use of that

4 good

Note: I don't try to put you off from buying a N900 but you really have to be sure you want it and know of it's shortcomings and not whine afterwards: why I can't play templerun or wordfeud. Just an example
Best wishes