There is a already a public resource that does this, but it's a bit bugged and it's written in spanish. It does have a login/register system and a basic class selection, but it need a few fixes and changes to work well. Search in MTA's Resources for "diversion" if you wish it.
Btw, actually, I'm currently making a 2.0 version of it, with much more improvements and massive changes, so it won't last so much