Networks

answer for Q#1 yes, SBS 2003 will allow you to share internet access and email access on your ISDN connection with proxy server 2.0.
Answer for Q#3. No Norton antivirus does not have a firewall as part of the antivirus program
Answer for Q#4 yes however you should familiarize yourself with the migration from NT4 to SBS2003 on the Microsoft SBS 2003 web page FAQ
Answer to Question #5. order the trialware SBS2003 from microsoft and try it out.

Now for an answer to question # 2. SBS2003 comes in 2 flavors. Standard and platinum. The standard edition has exchange server but no ISA server. The platinum comes with ISA server. ISA server is the Firewall/proxy server. . SBS 2003 also has an internal [not public] web site and sharepoint services [which is pretty nifty].
Now for the mail portion of SBS 2003. Exchange 2003 has to have Active Directory installed to work AND for "public" [internet inbound mail] a FQDN plus DNS services. Depending upon your existing mail services, you can use Exchange as a backend server with POP3.

There isn't a single manual for SBS 2003 as SBS2003 is a compilation of Windows Server 2003, Exchange Server 2003, SQL Server 2003 and for the platinum edition ISA Server 2003. Each ofwhich have their own set of best practices [which do apply to SBS edition].

The answers already given are correct. However, in your position, I'd do things a bit differently:

Get Win2003 SBS; for a small network it works well, and it's an environment you are used to.

Purchase a broadband router from D-link, LinkSys, SMC or Multi-Tech to share your ISDN connection. These units are exceptionally easy to set up, and offer pretty good good fire wall intrusion protection for yor network. These can be had for well under $100 -- money well spent is saved time and server resources.

Resist the temptation to install and use Exchange, unless you absolutely need the collaboration functionality. For simple SMTP/POP/IMAP/WEBMail access, you're better off to download the FREE express edition of iMail (http://www.ipswitch.com/products/ imail_server/index.html)

You'll need to set up a virtual server on the braodband router to allow incoming e-mail, but's that not hard, and is usually well documented in the manual.

Norton Anti-Virus is not bad, but my personal preference is Sophos (http://www.sophos.com). They are very corporate-centric, the tech support is top-notch (and available!), automatic updates to all your users is a breeze, and I've not had a minutes trouble with the software interfereing with any other programs.

I had to take charge of a newly installed W2003 Snt Ed. with no experience etc. etc.

In any case, Ive tried to setup RRAS (instead of ICS -- Internet Connection Sharing) for reasons explained in the source material I used. Something is still not working -- ahhh...

I would really like to know whether you have succeeded in your attempts. We are at the point of expanding the network to the rest of the premises. Planning, planning, planning...