Played this twice in the demo months - £12.50 seems very reasonable in current sale.
Any recommended DLC's - some look cosmetic, whilst others potentially useful...?
Similar thoughts to Jim99. But I think I'd miss all the areas tbh. I don't really play the story, I play MP with a few friends generally but I like each region and their uniqueness.
Land of Lions adds a water mechanic where you have to create irrigation from river sources for certain things like farms and animal pens (for increased efficiency) as well as fire stations. Apart from that it's got its own resources and needs but it boils down to being more of the same.
The Passage adds a heat mechanic as it's all in snow, you need to build things within range of a coal heater. Again its own resources, though it also wants stuff from the Old World IIRC. Also adds in the Hydrogen Airships IIRC (I think the helium ones come from the expanded New World in Season 4).
Crown Falls is the 'least' interesting as it's effectively Old World map 2. There is one massive island on it though which is nice for building on.
Cosmetic stuff is nice, but can be a pain IIRC. Ignoring skins for vehicles, the props you can put down block roads IIRC so you can't really design stuff with them in mind too much as it ends up handicapping you.
From memory, Ubi shafts you if you don't buy them all DLC at the same time, but things may have changed now with the Annoversary edition. I'm not even sure if they still sell the game with all the seasons only, but that's what I'd look at as it normally saves you money in the long run annoyingly. It will throw you more in at the deep end as there's more mechanics to learn, but it's not like you have to tackle everything all at the same time. For example, you'll start in the Old World and get to grips with the basic game there, then move to the New World and learn there. Then you can expand out to other regions.
I'd probably suggest playing your first game with no AI and no pirates, just so you can get used to the mechanics, then when you're comfortable start again with some AI and possibly pirates.