Is king Arthur worth it or should I buy him in European War

Arthur is very good, but it depends on what you are aiming for. For endgame, it's necessary to have generals with commander skills (archer/cav/inf commander), so Arthur would shine in an inf comm lineup.
In the early campaigns there is no need to focus on having a commander lineup, since free generals and the cheap coin generals (Moctezuma, for instance) are enough. But soon you will have to decide on one of the three categories. I suggest archer commander since they can avoid retaliation and you get some generals with both archer comm and leadership (the most useful skill in the game).
If you choose arch comm, then you can consider buying Washington, he's got the best stats among archers (and nice skills/perks, too).
Remember also that having gold generals (i.e. the IAPs and Alexander, Caesar, Suleiman, Li Shimin) will mean having to spend more medals to upgrade them, compared to what you would spend for purple or blue generals.

This thread by jonblend can help you compare the generals when they are fully upgraded. Check also the linked threads at the beginning of the first post.

As regards grinding medals, there are two princesses that give medals when you login every day, then there are "daily" missions (actually they refresh after 12 hours) that will increase in number as you progress in empire mode. When you get to 155 cities in empire mode you will have a maximum of 10 missions that give 20 medals each every 12 hours. Then you have the option of repeating one of the chapters in legend mode for medals (you get 3 tries every 12 hours, so it would be 90 medals every time if you repeat a nordic/greek trial or 120 medals for 3 decisive battles).