Waltorious: Just to clarify, I'm fairly certain that the Heroes series will not spoil anything from MM3-5. It wasn't until MM6 that the RPG games were set in the same place as the Heroes series, and while MM6 does have a few references to some of the earlier RPGs, there aren't any spoilers. They're more like little bonus details that you'll notice if you've played the earlier games.
Having said that I've only played some parts of Heroes 2 so I don't know the full stories of the Heroes series.
Theoclymenus: This all helps, thanks. I've read number of threads on this but I wanted to be sure that I wouldn't be spoiling the experience for myself by playing HoMM2 before M&M 3,4 or 5. I just wanted an idea of where to start. From what I've read HoMM2 + 3 and M&M 6,7 and 8 take place in a different world than M&M 3,4 & 5. It's pretty confusing to the uninitiated to be honest !
The joy of the Might and Magic series is that your characters are Joe Schmo from the backwoods barbarian people on a planet that isn't very civilized or cultured. But your characters are so great that they get involved in things going on between ultra-awesome-space-faring-super-beings. That's why things happen on different worlds. They're different worlds, but the same universe. The sci-fi setting stuff is what connects them all together.
If you play HoMM, you'll be playing in and around events of M&M 6 and 7. But if you play them out of order, you'll be able to fill in the gaps.
I wasn't so hot on MM1, 3, 4 or 5. I loved 2, 6 and 8. (i'll probably like 7 -- it's my next game on my Might and Magic list). Even though I've played them all out of order, the things spoiled aren't that big of a deal. The games really are more about the gameplay and the experience than the plot twists.
I'd recommend starting wherever you feel like starting (you said M&M 3 and HoMM 2). And if you get sick of a game, move on. I spent 11 years with M&M2 on and off. I did end up finally beating it, but it was a joy just playing it and finding all of its secrets. Some of the M&M games are really, really long (500 hours). And like you said, your time is limited on this earth. So if you get tired of it, look up its ending and move on.
It's a great series. I hope you enjoy it.