There are many different low level zones. Different races start in different ones. Some have to fight their way out, but most (Like humans in the Trollmire) can simply leave to the world map right away and ehad for another dungeon. The starter dungeons would pretty much be Trollmire + Kor'Pul, Crystal Caves + Shaloren Hideout, and the the frozen forest and blighted forest (These aren't the exact names, and I'm grouping them by location, there's 2 near each of the 3 towns of Derth, Elvala, and whatever the woodelf place is called. Some are easier to start in than others, and different classes have different difficulties in some areas and in general.)
Levels never repopulate. I don't think shops restock anymore either. Basically impossible to scum, except adventuring parties, but those are deathtraps. There's easily 2-3 times as many dungeons as you need to be properly geared and leveled to play through the game though.
As far as when the game picks up... it depends a lot on your playstyle and class you pick. Most classes have a lot of active abilities and a lot of passives. I like stacking passive bonuses and winning via mathcraft a lot myself, but playing tactically with the abilities is certainly viable. My best character so far was a Shadowblade; I combined probability travel (high level sustained wall walking magic) with stealth to warp in and out of combat and blow all my tricks on an enemy to take them out then fuck off and recover. If you pick the plain human you get a bonus category point to start with which might net you some more options, though some classes have actives based on different stats (Say, dex moves and strength moves) so learning both will kind of gimp you. You're also missing out on 3/4 of the abilities until you hit level 12 at least. You can gain levels pretty quickly though, a lot of the dungeons only have 3 levels, and if you go in a bit under levelled, you can easily gain 3-4 levels. I'm usually level 8 by the end of the 2nd dungeon. I usually beeline for the stairs, kill the boss and head for another dungeon. Less tedious combat, faster levelling, but you miss out on the gear in the vaults and scattered in the levels.
I'd suggest checking out the crystal caves to get an idea of what some of the other areas/enemies/bosses can be like. As far as class variety, you'll have to play into the game a bit before you start unlocking most things. Archmages and summoners both unlock really easily though. The first via a quest for someone on the overworld map, the second you'll get by accident sooner or later. Most of the other ones are specific dungeons/sidequests you have to complete, sometimes in a specific way.