If it's just bog standard Sage 50 Accounts then you'll not need SQL, as it's an old flat-file based system. In fact, even for that size of a business, and if you
were using any SQL-related database, you'd get away fine with the free SQL Express, no need to up from SBS Std to Premium.
Moving Sage Accounts is dead easy - create a shared folder on the server, move the existing data folder to it (default existing folder is c:\programdata\sage\accounts\20xx\Company.000 in recent versions).
Then open the c:\programdata\sage\accounts\20xx\COMPANY file in Notepad on each client machine, and change the text there to be the Sage shared path of your new server.
Suggestion: Don't create one single share for everything and dump your Sage company in there; create a separate share for it. That way, it (a) stops people navigating to the folder and saving/deleting stuff in there accidentally, and (b) can be secured better - you could create a security group in Active Directory called 'accounts users' and add only the people you want into that group - you then set the share permissions so that only that security group can access it.
I don't have an outright answer to the OP as my initial experience in SBS was SBS2000 and that was watching another guy set it up then I looked after it from there. It was some time after before I commissioned one myself.
Over the years I've inherited a few setups where someone with a bit of know-how has tried it themselves and got about 70% of the way but little things (usually Exchange-related) have held them up. To date I still assist a couple of I.T. companies with SBS installs as there's random quirks that always stump them.
If this is a system in your own workplace then give it a go, you'll always be there for troubleshooting and will have plenty of time for working with it. It'll be a great way to learn, rather than watching someone who only makes the correct decisions every step of the way
First steps - do it over a weekend; log into your router and turn off DHCP on it (the SBS will do this from now on). Install the operating system and one of the first steps will be where the server finds the existing router and gives itself an address based on that.
For jobs where I put a new server in while people have to stay productive, and no domain server exists, I turn off DHCP on the router, then set static IP's on all machines, so they can all work away while I bring up the SBS and restart it without affecting anyone. At switchover, bring everyone onto automatic IP's again, then join them up to the new domain