ESO combat is very active. Healers are expected to deal damage and debuff enemies, damage dealers will constantly have to consider using defensive abilities. Fast movement, blocking and dodging all consume stamina so there is a limit to what you can do which requires you to be strategic with your resources (tanking is a very strange beast).
ESO has a lot of content, mainly because all content released since day one scales to max level and much of that content still gives very relevant rewards.
Building a character in ESO . . .well frankly it is not clear at all. There are 5 classes, soon to be 6. Every class can do everything. Most classes have 5-6 different ways to be played that are viable for even the hardest content but if you are not interested in veteran trials and veteran HM dungeon bosses, the possibilities are endless.
On how a build is made: race matters quite a bit but beyond race and class you also have access to weapon skills, armor skills, guild skills, pvp skills (which are also broadly used for PvE; the Assault Ultimate is used practically by every healer and tank in the game) as well as options like being a werewolf or vampire. Everything has its used.
You can certainly push yourself by soloing group content in ESO. The premier ESO solo challenge is the Maelstrom arena. Imo it is as difficult as the harder of the Mage Tower challenges . . . when played so you can just complete it. It has leaderboards though and many ways to make your life harder
ESO can be somewhat restrictive with how you build your second or third spec. Still, you do have a lot of options and there are some playstyles that pair really well (magicka dps+healer is very easy to build for both classes and you will only have to change Mundus on occasion). Gear is anything but restrictive; if anything ESO sets are an endless opportunity to create new builds.
ESO has NO faction barrier for PvE if you buy the All Races All Factions upgrade which is fairly cheap to get.
The game is B2P. You will buy the base game+latest chapter. The game has numerous DLC which you can also buy OR you can pay a sub and get many perks PLUS all DLC for free. But there are many variable ways to buy the game and its components. As for how limited you'd be with just the base game? Well you seem like a player who is interested primarily in group play so I'd have to say you'd be very limited. The harder dungeons are all in DLC and the base game has only three trials (ESO raids). If you liked solo play more I'd tell you that there are many months of constant play of content just in the base game though.
Also, ESO housing is addicting. Stay away if you value your wallet!