I asked my Monday night players how they felt about switching to a different rules system for the new campaign. I explained that I really wanted to run Basic/1e D&D using one of the retro-clones, but my suggestion was met with a resounding, "No, thanks." The guys know 5e and are confident playing it, so they have zero interest in learning a new game (even if it's sort-of the one we used to play 40 years ago).
I like core 5e well enough so I'm not terribly disappointed, but I was jazzed about trying something different (especially for this campaign, my adaptation of B2 Keep on the Borderlands). I haven't played/run AD&D since the late 80s—going back to that system or even Basic sounds like great fun to me, but it's not the hill I felt like dying upon.
Fifth Edition it is, then. To capture the feel of Basic/1e under the 5e framework, however, I established the following character creation guidelines. Because the Caves of Chaos are filled with lots of enemies, I also boosted some of the class abilities to give the party a little more endurance and enable them to press on longer without stopping to rest every other combat (which might be difficult or even impossible during an attack on one of the humanoid lairs).