I would start with MM6 and work your way through 7 and 8 ( MM8 got posted up on GOG just today), then finally onto MM9. You can find MM9 on ebay.
As far as toughness goes, easiest to toughest is as follows:-
**Easy** MM8, MM9, MM6, MM7 ** hard**
When I say easy, its still a challenge, just not the monumental challenge that MM7 is.
All of the are great fun and will take hours to play through.
You can also find the strategy guides for these games on ebay , and whilst they get progressively less detailed as they go on ( 7 is less detailed than 6, 8 is less detailed than 7)
they are still very good guides and very useful.
There are a number of online resources for all of the MM games, and I simply downloaded and printed maps, solutions and quest guides and made my own versions of the strategy guides as well. I didn't need to keep copious written notes for any of the games ( MM6,7,8 &9). But note, these are the only ones I've played.
They are all terrific games, and if it weren't for the fact that MM8 became available off GOG today I was going to go and try MM4+5 ( the Xeen story line).
You are in for a real old school treat if you play these games, and please note that MM6 should be played first as its a nice middle of the road difficulty. MM7 is brutally hard, but finished it again for the 4th time just before posting this reply. MM8 is lot easier than 6 or 7 mainly due to the excellent and unique grouping system. HINT:- Join the dragons and NOT the dragonslayers !
When you play MM9 be sure to install TELP's latest patch ( 1.4 I think) just look for it on Google, as this fixes a few issues, and also make saves prior to making promotion decisions as they are irreversable in MM9. MM9 is the only game with a different and slightly confusing promotion system, but just save and pay attention. Whilst un-finished in some graphical aspects, it is still a fine game and very, very enjoyable.
Any questions, post them here and I'll try and answer them for you.
Post edited March 23, 2011 by Fortysixter