This post might sound a bit blunt in parts but I'm just trying to be candid. Also, if something seems patronizing, it's not meant to be (just trying to make it as simple as possible). Now, disclaimer outta the way, let's see if I can shed some light.
1) Monk. I have both, heavily developed (75 monk with 1695 AA in anguish/demiplane raid gear and 71 Berserker with 1554 AA in Guk chain (read: group) gear). The monk outdamages the berserker due almost solely to the disparity in gear level (2.0 monk with Cryptwood Tonfa, among other toys vs 1.5 zerker). The AA after about 1200 or so are a wash (due to diminishing returns) but the levels do matter. Plus, the monk is a bit sturdier and easier to heal and brings Monk utility (like pulling and such) to the table. I love 'em both but if I had to choose between the two, I'd go berserker (haha, switched it up on ya
due to pure DPS and being a force multiplier (berserker buffs)). In a 4some with a tank & healer merc, I'd want as much damage as possible. But for the group you describe, go Monk since you already have one at the level you need.
2) Slot types have to do with something called 'augmentations'.
Look here for more information.
3) Focus you should be familiar with, since they were added during the Luclin era. They used to be called 'focus effects' or 'foci' (in plural) and it used to be for casters only. At some point, the old system was merged into the focus system so effects like 'Vengeance' became just straight ATK buffs and semi-new effects like 'Cleave' came into being. So now, both casters and melee have focus effects they clamor for (though casters moreso than melee). You'll also see terms like 'mod2' and 'mod3' thrown about. These are all the various effects you see listed (primarily for mod2s) on the far right side of an item's info window. Stuff like Spellshield, DoT shielding, Avoidance, Clairvoyance, HP regen, Mana regen, Damage Shield, all of these things are mod2s/mod3s.
Purity is a trait that was introduced in the Buried Sea expansion. It has to do with the power source slot you might have noticed. Power sources are basically buff batteries. They add (and subtract, in some cases) stats and mods to your gear while the power source (PS) is equipped. But, while the PS is equipped, it drains like a real battery until it gets to 0% at which time it stops giving the bonuses. Purity determines how much of a PS's buff you get. More purity = more buff, basically. How much a PS is drained depends on how many items are drawing "power" from it. So, if you have 10 slots pulling power, you'll only get like 10 or so consecutive hours of "buff" from that power source. Purity determines how much of the buff you get. It's a percentage (so 50 purity = 50% of the PS's bonus) so higher is always better. Look
here for some more basic info.
4) Mercenaries are available in the Plane of Knowledge from the various Mercenary liaisons you see. But, keep in mind the restriction. You can have any race tank or healer you want EXCEPT if that race does not inherently have clerics or warriors available. So while you can't have an Erudite tank (because they can't be Warriors), you can have an Erudite cleric. So no Barbarian healer merc (no clerics) and no Erudite tank (no warriors). But, mercs do not have racial benefits. They're just skins on a robot. So your Ogre tank merc WILL be stunnable from the front. Pick a race you like the looks of that has access to the role you want (warrior for tank, cleric for healer) and have at it. Make sure you have enough coin to pay for the purchase AND the upkeep though.
5) Most servers aren't teeming with people anymore. As far as I know, the most populated servers in EQ right now are Antonious Bayle (Euro server but there are more than enough folks on during US prime times), Maelin Starpyre (seemingly the server everyone transfers to for the highest end content), and Stromm (perhaps the most new player/returning player friendly server). The servers with "dead" populations (or rather, less than ideal player populations) seem to be Zek, Firiona Vie (the Hotel California server), Drinal and right now The Combine (though it's being merged with Druzzil Ro supposedly next week...assuming they don't delay it.....again). I can say Fennin Ro isn't exactly Guantanamo Bay either.
a) Not really. Once you hit 70+, you'll find that while pulling can be good, it's often unnecessary. In fact, in most situations, a bard will pull better than you do (better and/or faster). Post-80, add Ranger, Rogue, Necro, SK, Enchanter, and Cleric (situationally in most of these cases) to that list. You'll find that your monk can lull now but after 80, there are mobs that are flagged immune to that ability (or extremely resistant) but not to things like Ranger's Harmonious Arrow or Bard tactics or even pacify. That's probably why you hear people say monks suck now (they're wrong of course, but such is the way of lemmings and parrots).
b) Any level appropriate zone will have defiant armor drops. The closer you are to the recommended/required level of the armor you want, the better your chance of getting A drop (not necessarily the one you want or need). It might be easier to just farm up some plat (it's a lot easier to do than you remember) and buy it since those tiers of armor are typically the cheapest (Elaborate, that is. Elegant can get pricey, depending on armor type and slot).
c) None right now. The only flags or keys you'll need at this point aren't attainable by you now. Those are the Secrets of Faydwer and Seeds of Destruction flags and keys and chances are good your monk in his current form would be shredded (1-2 rounds, tops) by many of the mobs you're required to kill. Everything you remember being locked behind a flag or key is now open by level (yes, even Vex Thal). Heck, you can even get level 1 characters into the Temple of Veeshan and (I think) Sleeper's Tomb. Your best bet is going to be looking into starting your characters' 1.5 epic quests (assuming you already have 1.0). While doing those, you'll get decent to good xp at your level, tackle challenging content that's new to you, die a whole lot (but learn a whole lot as well) and progress your characters really quickly (far faster than you remember). By the time you hit 75, you should have epic 2.0 done (or almost done), 300+ more AA, and much better gear. You'll find a lot of upgrades in bazaar for dirt cheap (look up '
Adornment of Frenzy' ). Look in bazaar by slot and you'll find tons of stuff like that for under 500pp on most servers (seriously!).
Hope this helps and welcome back! If you're on Fennin Ro, send a tell to anyone in Rendition and we'll see if we can help ya out. Can probably group with ya too since I have a cleric and wizard at 65 and a rogue at 67.