First of all, great choice. :)
Back in the old days I started with Might & Magic IV Clouds of Xeen. It's a great game to begin with and I don't think that it depends much on the storyline of its predecessors. Infact, MM4 and MM5 (Dark Side of Xeen) can be seen as one game (called World of Xeen), as there is the option to merge them, so that you can travel between both worlds ingame. Personally I think that this is the best you will get out of the Might & Magic universe of games :)
I'd say, get a hold of both games, do what's necessary to merge them (don't remember the whole procedure, but it can't be more than copying files together) and start playing :)
Notice, that the merging of games was officially intended and not a fan-made crack or something like this. It's quite exceptional, I'd say. Haven't seen this much with games.
You can also start with MM3. This game stands alone however and as I remember it is a great deal harder. I haven't gotten through this one yet, MM4 and MM5 however I did beat.
I don't have any experience with earlier games unfortunately. Maybe I should look into them some time. :)
If you like these kinds of RPGs, might I suggest some other titles? (Don't get me wrong, Might & Magic IV/V is top-notch) I'm sure you'll like Stonekeep and Anvil of Dawn, maybe even Ultima Underworld or The Elder Scrolls: Arena.
It's a real pity that step-by-step dungeon crawling RPGs of the western world like M&M are quite extinct. What comes closest to them nowadays might be John Carmack's Orcs and Elves for Nintendo DS, albeit much simpler in game design.
Hope this helps,
locutus
for more Retro-related tidbits follow me on X under @ChrisR3tro.