So sorry I missed this question!
Technically 1HD = 1d8. But I say 5 just to get a high average.
+1 or +2 tacked on to the number of HD is honestly something I’ve never figured out on old school games. Does the monster get a flat +2 hp bonus, or +2 hp per HD? I’d say it’s up to the GM. A dial you can turn on encounter difficulty
It also affects to hit bonus. Google THAC0 monsters and OSE. A monster with 3HD has a +2 attack bonus. But a monster with 3+1HD has a +3 attack bonus.