Celebrim
Legend
What kind of guidance is there in judging challenges in these OSR games?
Almost none. DMs in the old days relied entirely on experience. If you don't have that experience, you'll need to work out the math for yourself - what is the AC of your PC's, what is the chance a Bugbear will hit that, how many hits can the party take before it starts to crumble, how much damage can the party unleash in that time?
XP calculations are also rely unreliable in BECMI/1e/2e. I made attempts to reform it back when I was DMing 1e, but its a daunting task.
So, for example 5 bugbears might be a pretty good encounter for a party of third level characters, if the party mostly had AC 3 or better and their were 6-8 party members. If you got two part members, and one of them is a mage with 9 AC - 5 bugbears (much less 8) is going to slaughter them.
As a player, the most important thing to do in old school was to get your AC up to the point that most monsters only hit you on a 20. Then you were golden, and very few things in the book were really even a challenge. Those you relied on having a high level mage to dispose of quickly before they could become a problem.
You're really on your own here.