I am interested in running a D&D game that take place in medieval Europe. Does anyone have any experience doing so? Any recommendations for modules or 3rd party products?
Well, it's not D&D, but the historical GURPS books are generally very highly regarded, and they are fairly rules-lite, so you will be able to get lots of use out of them...
Of special interest are:
GURPS Middle Ages 1: Britain in various time periods.
GURPS Low-Tech: equipment -
not just weapons, but also anything else of use to the medieval adventurer.
GURPS Arabian Nights: For the neighboring regions to the south - as centers of high culture and learning, they will have an indirect influence across Europe...
GURPS Russia: Medieval Russia in all its glory.
GURPS Vikings: Those friendly neighborhood raiders.
How do you handle magic? I was thinking of making it low magic but I am not sure how.
Well, one option would be to ditch the D&D spell slot system entirely and go with the one of Call of Cthulhu d20 - that should make magic pretty low-key in most cases. Decide for yourself whether or not you want to use Sanity...
Another way would be to steal liberally from GURPS Cabal and assume that all spells add three to their spell level -
unless they are cast with certain appropriate materials, in certain places, and so on. Make the characters work for their spells.
How do you handle races? I was thinking of making the various races sort of hidden secret societies.
Either ditch them or keep them as NPC races. Humans should predominate, and other races would be too widely distrusted in a medieval society.
How do you handle classes? I was thinking of adding benefits to the NPC classes, and modifying the spell using classes.
Get Fading Suns d20. That book contains classes like nobles, priests, and guildmen that don't have any supernatural powers - but they have plenty of
social benefits that make them the equal of standard PC classes (unlike experts and aristocrats).
Come to think of it, the theurgy and psychic classes from that book might make nice low-key magic-users as well...