I'm still fuzzy on some of the details, but yes the requirement to get in is lvl 17 and one of the first dungeons completed in the main storyline quest.
You want to stick with the main storyline because it is what unlocks everything in the game for you, from dungeons to trials/raids. It also gives a boatload of EXP.
You don't use your gear in it, which makes it nice to not have to worry about that. It's basically mimicking a rogue lite type of dungeon. You have 2 stats that carry over, even if you start over on floor 1 or keep progressing which is aetherpool, one is Arm for Attack the other is Armor for defense. this starts at lvl 1 and when you find silver chests in the dungeons, they have a chance to give you +1 to either of these. It goes up to +99 total for each and makes your character definitely stronger.
Floor 1 starts at lvl 1 and you level up very quickly, they are not related to your actual level outside of the dungeon.
If you start at Floor 51, you'll always be lvl 60 and that is the cap for the dungeon, the subsequent floors just get harder in terms of mob health and damage.
The floors are always randomized just like diablo 2, which is awesome.
I think you can exp grind this to max level if you wanted to. You only get exp if you finish the chunk of floors, it's broken into 10 floor per chunk. So if you make it to floor 10 and wipe on the boss, or floor 9 and wipe, you get nothing, but if you beat floor 10, that chunk is done and you are rewarded with currency and experience and sometimes loot if you found hidden chests in there.
It's probably very good for grinding 1-60, at 60+ there is another deep dungeon just like it called heaven on high, which is for lvl 60-70. Like anything though if you grind it too much, it can be a burn out so I'd say do it in moderation.

It is extremely boxer friendly because it will allow you to try any comp, you don;t have to have a tank or a healer for this dungeon. It allows 4 dps if you want and you can still succeed. It also lets you try a new class without having leveled it up, and experience it at lvl 60 to see if it works well. It's almost similar to maybe making a trial account in WoW with the lvl 100 boost, just so you can see if the character is good or not.

Overall it's been fun!

Also!! I just finished Floor lvl 170, only 3 more chunks to go and complete. I think the bosses on Floor 180 and 190 may be a little tough but we'll see!