Unfortunately I can't speak directly to the viability of the comp, but I can say that as a returning player myself using ISBoxer and being able to play 5 characters at once has been well worth the investment. Especially since Classic is only $15 per account (no expansion costs, etc). Lots of fun and adds a whole different layer of engagement and entertainment for me. Especially getting to gank some people on my way dungeon grinding lol. Controlling a whole party is OP to say the least.

I will say though that while I think Druids are generally regarded as the worst of the 5-man multiboxing dungeon healers, I think if you're patient it will be a comp capable of clearing the dungeon content (with 1x bear tank, 3x dps, 1x resto). Will it do as much damage as some melee cleave or spell cleave teams running Warriors/Mages/Locks, probably not... but if you're sold on Druids I think it will be a fun experience. Sometimes it's about the experience in my opinion, and I can imagine a boomkin/Druid squad being fun to play and capable, if not the most capable.

I'll let more experienced people chime in on what might be better for you to run, but I highly encourage you to get in the game. Costs are well worth it in my opinion and even for someone with zero experience I've been able to get up and running with ISBoxer in no time. As someone who's done it previously I'm sure you'll be right back at it in no time.