MacOS – How to replace Microsoft Small Business Server 2003 with Mac Mini Server (Mountain Lion)

With Lion this is possible, but I think there is a limit of 2 sessions, and you don't need to be running the server tools. When you screen share a Mac that has it enabled, you can choose to either just take over the screen as per the scenario you are used to, or move to the "virtual display", where you can log in as a 2nd user.

Many (particularly PC/Windows) VNC clients are getting tripped up by this new behaviour and do not connect at at all at present, so you might need to careful with your choice of client.

Edit: I think the 2 session limit is to provide the ability to allow an administrator to login on the 2nd session to work without interupting the normal user, I would test whether more than 2 sessions are available, but I can only get it to work from a Mac client using screen share via the finder rather than a specific VNC client app, and I only have 2 Macs...

I've never tried any of the Mac Server products. But AnandTech has a pretty good in-depth review of Mac OS X Lion Server. A few of the conclusions:

...server administrators will be happy to know that, in spite of its deeply cut price and consumer-friendly distribution method, Lion Sever stacks up favorably against Snow Leopard Server.

and

OS X Server is most useful in a handful of different scenarios: the first is that you have a small network that’s in need of a full-featured but easy-to-manage and simple-to-license server product. The second is that you’re managing a network of any size that used to be all-Windows, but hosts a growing number of Macs (this is often the case in education, for example) - OS X Server knows that it’s going to be finding its way into a lot of Windows houses, and as such it integrates fairly well with existing Active Directory setups. The last is that you have a bunch of iOS devices flooding your network and you have no idea what to do with them - iOS management may be Lion Server’s ace in the hole.

(emphasis mine) Seems to fit your intended usage pretty well.

About your other question:

Besides, and this is a bonus question, do you have some experience in using Lion server to manage centralized Xcode development? (we currently use our own MacBooks and we keep in sync using Git, I'd like to know for different approaches based on OSX Server).

Keep using git, use the OSX server as central repository (maybe consider buying GitHub Enterprise or using an free/open source alternative for managing your git interaction). Install a CI-tool (Jenkins or similar) on the server and use it to create automated builds. Should work quite nicely.