Im tired of using IIS6 that got some problems running MySQL5 crashes some of my scripts.
You will be going away from out of box upgrading the standard SME 7.x to MySQL5. This is not recommended for a n00b, infact as I recall reading is not recommended. Try searching for each of you queries separately - i think when you look at mysql5 you will see in one of the recent sticky posts about advice that a sme guru has voiced this quite well.
My suggestion to you is to read the documentation as per JoshuaR, and remember search is your friend! The SME server is a brilliant distribution and the only cost is your time - well worth the effort.
im looking for a solution that can handle same way as my windows 2003.
You can get better results with SME for all the same tasks you have listed, however this will not be in the same way as windows 2003. Thankfully!!

But you will find the server-manager excellent for most common tasks as available on windoze, just without the server resource requirements / overhead and horrid licensing!
Can I add multiple domains with each FTP accounts, Email acounts ect...?
Like a webhotel, where people can get access to there own site?
Maybe addons ect..?
Again, as stated in earlier replies - yes!
If yes, is there a guide?
Again - yes, as per JoshuaR post.
www.wiki.contribs.orgI would suggest you read both the admin and the user guides. Then read them again! Play with the distro and get back with specific queries. The community here really is supportive and knowledgable, and willing to assist with specific problems, however really broad queries tend to get the standard rtfm result. Don't take this the wrong way - it really is the best way to start.
After reading the docs download the iso and install your server onto a test machine. You'll be amazed at the ease of use once you get started.
Good luck, and again let us know if you have any specific queries.