The Sigil
Mr. 3000 (Words per post)
Not sure if I can Summon @The Glen but if you want 5e plus Mystara, I'll just drop this link here on his behalf: https://www.rpgmp3.com/mystara-players-guide/ for a free PDF of the Mystara 5E Player's Guide (Can't beat that price!) - formatted so you can print your own on lulu.com or similar if that's your thing.Regarding Basic, I never played it.
I am curious about its Mystara setting.
I prefer to see a 5e version of it.
I have no interest in old school gaming mechanics, except how its thematic flavor might be relavent to the setting.