Have a look at this thread comparing stats of the characters:
image from above forum post
Note that the difference in stats is really pretty minor, and by equipping materia to any character you'll see more variation in stats than you will between characters, if I recall correctly. Additionally the Materia system allows much more versatility than just making someone a "healer", you don't have healers so much as "people with Cure and Revive", so it's easy to slip healing into any team.
Aside from their limit breaks, all characters are basically blank slates when it comes to their role in battle. Note that while cloud is the second best at magic...he's also definitely the best overall and is easily suited to any role. Basically pick a team of three favorite characters based on their personality/looks/limit breaks/weapons/whatever you please, then assign roles as you see fit. Since Cloud is basically a given and too overpowered to dedicate for healing, my healer was always just "Not Cloud" or occasionally "Not Cloud #2", though I usually gave multiple, or all, characters Cure.