First, spam has less to do with what client you use and more to do with how much crap mail is coming to your emial address. If you have the same email address using a different client to read that email, expect the same ammount of spam.
--edit, yes you can filter it, but if your anal like me, I don't WANT to know its coming to my account, but then filter it out - I just don't want there to be any period. More on how I have been spam free for 2 years below...
1. Yes.
2. Well, I think the answer is yes, however I have IMAPI email so I download it off a server, meaning my mail is always there when I move to a new client. I believe you can save your outlook mail to a pst (or whatever the extention is) and then import it.
3. For me, I started using thunderbird over Full Outlook - the main reason being, I didn't need or want all the superfulous crap that was on Outlook for my home email. The intereface for Thunderbird, is slick, clean and useful - had all the features I wanted, none of the ones I didn't.
If you really want to be free of spam, can I give you a few pointers? I've been free of ALL spam (i.e. unsolicited or unwanted mail) for 2 years now, and here's what I recommend.
1) Don't use hotmail, or and msn account as your main account - they are prime targets for spam.
2) *DO* use a hotmail or yahoo account as your PUBLIC email account. If you have to register for a site and it needs your email address, or your some business site needs it, or other people that are not people you want to be getting mail from - give that address. That way YOU can check it if YOU need something from it, but otherwise you can just ignore it.
3) Then, after 2 *NEVER* give your private email out to anyone you don't trust.
4) Finally, I'd strongly recommend paying the annual 30-40 buck fee and getting a personal email account not associated with a huge commercial isp (like MSN, or Earthlink or AOL). For example, my email is --edit--(wow I'm tired, I can't believe I posted that here) The actual service is run by terbolic. For 40$ one time a year, you get their own spam protection and filtering, you get a fairly large grouping of features - for example, it is IMAPI so its like your own exchange server, messages are stroed on the server - up to 30gigs or something I think (I've never run out of space) and then your local email client pulls them from there. Both their incoming and outgoing servers are SSL encrypted. Becuase its IMAPI it means that you CAN use a local client to get the mail, just like a Pop3 system - but you can also check your email via a webclient from any browser anywhere as long as you have your username and password. Lots more freedom.
Check them out:
https://secure.terabolic.com/signup/indexsingleemail.php