1. FR, T1 (Full/Fractional), ATM (less common these days). ISDN is usually used only as backup.
2. MGX is not a wan router. its a wan switch. I am not sure exactly what you are looking for on this. If you require port density for a medium to large scale, the 3800/3700s are a good combo, as well as the 7200, 7500. (DS3 performance)
3. Features - Scalability, number of NM slots, PA (for 7200), wic support, horsepower to support the type of circuit terminating on the router (like multiple DS3s if needed)
Specifically look for PPS, which will give you an idea of the perf. of the router and yes definitely invest on something, thats gonna stay with Cisco support for a while.
Sankar Nair
UC Solutions Architect
Pacific Northwest | CDW
CCIE Collaboration #17135 Emeritus