The Elder Scrolls Online – Identifying Skills Guide

See if I can answer all of your questions.

What exactly is the point of leveling a skill?

To gain access to higher level skills.

I sure do get that you can unlock new Skills and perks but does It do something else?

No.

Will you gain more Armor/Damage if you have a Higher rank in a certain skill in ESO or is it solely for the sake of unlocking new stuff

Yes. Not sure how much it scales.

Also if you Level up an Active Ability, will this ability get stronger or is it also just for upgrading an ability. If it's only for upgrading, then why can I further level the upgraded skill (I've noticed that you can upgrade an Ability only once.

Yes. Not sure how it scales though. I believe they are planning on implementing one more morph to the skill lines in the distant future.

Another thing I'd like to know is: What is the max level of an Ability/Skill

Rank IV > Morph Rank IV. Max Level of weapon/armor/class skills is 50.

As of right now (end of May 2015) there aren't any DLCs released so far, since Zenimax Online Studios wanted to release the game on consoles first, then start releasing DLCs (which they've been working on since last year).

So far there's no information available regarding prices, but they've already said, they'd release more information as well as a roadmap soon (i.e. at E3 2015), which will start in about two weeks.

However, since a subscription will also net you crowns to spend (the same amount as if you'd be purchasing the 15 $/€ package once a month), you can also subscribe now and use those bonus crowns later on to buy finished DLC.

The first DLC to be released will be access to the Imperial City, although they haven't stated whether that content would be free for all or indeed be the first DLC (since it's connected to the PvP/AvA endgame).