Yes, you will find Excalibur stuck in a rock in Eel-Infested Waters. You can get ALL the artifacts as random drops, but but only provided that you haven't already taken the placed ones. (If you take the placed ones first, you don't find random copies) This means you can legitimately get two versions of Excalibur, Mordred, Igraine, Morgan and Guinevere, which I find extremely useful for my style of parties. The only one of those that I don't have much time for is Excalibur – I find Hades and Poseidon to be the best fighting combination provided that you both have an item "of Regeneration" to counteract Hades' hit point drain, and have taken one of the mods that get rid of the bug that decreases speed when use two weapons.
The game allows you to find 13 artifacts in total before it stops giving them, so the way to do it is to get your 13 random artifacts before going after any of the placed ones. That way, you can get 18 artifacts for your party.
I too have had to replace my computer recently, which entailed moving to Win 10, and I didn't know if I could install the games from my old MM6-9 disks, so thank you for that information. I guess GOG is going to get a customer! However, I also want to see if I can transfer some of my old DOS games and run them under Dosbox. I want to see if I have enough digital dexterity left to play Eye of the Beholder and Legend of Darkmoon again.
I'd be very interested to hear of your experiences with the M&M games though, if you'd care to share them!