You need to double-check that you are running a protocol and security level supported by Win Server 2003.
Ultimately I would encourage you to not have a direct VPN in to your server. Ideally a user would VPN into your router and then access the server via the local IP. This allows you to more easily have multiple servers running on your network that outside users may access and it allows greater granularity of control for access. For example you can assign a user a specific VPN IP instead of just a random IP from a pool and you can grant them access via FW rules to specific IPs.
Win Server 2003 supports PPTP and L2TP. SSTP and IKE2 are not supported.
MS-CHAP, MS-CHAPv2, PEAP and few other authentication protocols are supported.
As far as I can see, this should work.
As for this suggestion, the whole idea behind RRAS server backed by Windows Active Directory is easier administration. Everybody already has defined access/rights/privileges, they just need to clear VPN verification and that's it. I tried doing as Revelation suggested couple of years ago, but it involved creating parallel security organisation. Or maybe I'm not aware of the way to get ROS to use AD created users/groups for authentication.