This is mostly a single player game, you have some horde MP available for when you want to play it. The game is a one time fee to play, there are some upgrades you can buy(DLC) planned for future expansion of story but nothing announced definite so far. The online part does have a store you can use real money to buy stuff to help you do better, but isn't required to be used. Now as to needing the first games...not required at all, but you will not know the enviroment or characters if you skip the first two games. The first game Dragon Age: Origins is more an older harder core RPG, with an unvoiced main character, stats to worry about, click to move and pause and play abilities. In DA2, they tried to make it a bit more console shooter attractive but kept stats, while getting rid of some of the control for companions and skills. While DA2 is different from DAO it is still a pretty good game, got blasted because they reused dungeon systems by making all the segments of dungeons modular and would just reshuffle and occassionaly recolor segements for pretty much all dungeons. plus enemies have a habit of coming in waves, often dropping from sky to attack you. They did learn their lesson a good bit on the DLC they made for DA2, which is in many ways better than core game.
For DAI they kept some of the DA2 elements, got rid of stat control, but brought back more cusmization of companions and instituted a more open world style of play, that people say is like WoW... I don't play WoW so I have no clue about that.
So, Yeah you can play by yourself, no monthly fee, and you can skip the first 2 but you will miss on a bunch of back information. If you do decide to play DAI, I would head over to DragonAge Keep and https://dragonagekeep.com/en_US/ and set up a world state or at least read up on some of the story choices from the first two games so you may have an idea of what is going on.