This could best be answered by looking up the actual spawn tables in the source code, which if I recall correctly work something like this: you can specify a range of floors, and also specify how the distribution over that range works, ie, they can first appear on d:1, last appear on d:4, and get more common as you go down. Or they start on d:5, and end on d:9, but are most common on d:5 and decrease as you go down.
But off the top of my head, the rough answers are this: Rats/bats and other popcorn are rather common, and occur throughout the entire game. Rats and bats are only not popcorn to a level 1 character, but quickly become non-threatening.
Orc packs (with mages/priests) can start on d:3 but are more common around d:4 d:5, and later on can appear with orc warriors as well.
Ogres are usually first on D:3 as well, I'm not sure if they can spawn on d:2, but I think it's possible.
Uniques are fairly random, there are none on D:1, but several are possible starting at d:2. Early game uniques that are well known for being dangerous are Grinder and Sigmund, some other early ones are natasha, terrance, Jessica. Full list here:
http://crawl.chaosforge.org/Unique (not all of those are early, of course).
Gnolls can spawn on d:1, although I think there's some restrictions on their weapons until d:2. They're pretty common for the first several floors and eventually get gnoll shamans/sergeants included in their packs.
Traps seem to less common these days as a number of them have been removed, but random teleport/shaft traps are still a thing.
Potions and scrolls: Quite a few, of each. I'll let someone else go over the object statistics if you want precise values, but generally speaking identify/remove curse scrolls are the most common, and for potions curing/heal wounds. I'm not sure if you'd want to try to include 10+ scrolls and potions into a D&D campaign, so I might limit each to just the 3-4 main ones. That's subjective, of course, but I'd vote for scrolls of identify/remove curse/blinking/teleportation, and maybe enchant armor/weapon. Then potions of curing, heal wounds, might and agility. If you want to get really involved you could include mutation, but that's a lot of weirdness to handle. It would also be rather easy to cut out identify/remove curse if you want to keep things simple.