You only need to buy the 4 main civ packs which might be on sale on Steam for £9.99 sooner or later for all 4 (thats the usual price in the Steam winter sale so should be the same in the summer sale. The rest of the content you can earn the ingame currency (Empire Points) to buy them with just by playing the game but you need to get all 4 main civs up to lvl40 first this then allows you to earn the Empire Points to buy the content very quickly if you play solid for a month when you have reached lvl40 you can buy all other ingame content for nothing (crazy economics from MS but there you go they are giving away their own premium content for free

).
Most of the game can be played Solo anyway you can grind out all the missions & content that way will take around 200-300 hours minimum but personally I had a great time on this game over 2 years spent around 500 hours on it overall

I have some friends who spent over 800 hours on it
Co-Op is not really required at all although you earn more XP that way but you really need another newbie player & beware the gold traders as they all charge rip-off prices for impatient players just buy some Premium Civs then earn the XP to lvl to 40 & start earning the Empire Points to buy it all for free.
If however you do not want to wait for a Steam sale to buy the 4 main civs for £9.99 then its going to cost you around £60-80 to buy them & the Booster packs your best bet is probably this pack:
http://store.steampowered.com/app/201969/
Its only £22.99 & you only get 3 civs not 4 but there is enough content here to let you earn the XP & Empire points slightly slower but otherwise you need to wait until/when/if MS have the 4 civs sale offer again where they give you 1 free Civ if you buy the other 3 together.
Avoid buying the Pro Persian & Babylonian Civs as their missions are crap & you get no LIVE achievements with them either so best to buy them when you earn the free Empire Points ingame after reaching lvl40. There are still a lot of players online but most of the game is Solo based anyway some of the harder missions require Co-Op but most of it is doable on Solo I do not think the servers will be going anywhere for a few years as so many still play daily so you should get plenty of time on this as MS made a huge mistake when they cancelled development in letting you quickly earn the Empire Points to buy all the Premium Content if you already owned at least 1 Premium Civ!!