I can't give a lot of advice, but I do know this!
If you want to develop modelling skills that are generally useful not only for SRB2 but also for other things in the future (definitely recommended! you never know where you might go!) you could try picking up Blender. Blender is a 3d modelling software with a really high skill floor (can be hard to learn it at first) but it's really powerful. You'll be able to make md3s in there, but I can't detail the process for ya I'm afraid!
If you purely only have interest in modelling for SRB2 and don't plan on ever making or editing models for much else, you could try MisfitModel3D. It's a lot more limited and a little easier to pick up, and it will give you everything you need for SRB2 I think, but... it doesn't have so much use in more modern games.
Definitely give it some thought, it's an important question to ask yourself when you pick up a new skill. "Where do I want to go with this in the future?"
Modelling ain't gonna be a cakewalk either way though, it's definitely an art form that will take practice to get good at, but I bet you have it in ya!