1 Yes, albeit you'll need to know other people to play multiplayer because servers are usually empty.
2 You can of course choose all your loadout (ie ship, primary weapons, missiles), and even those of your wingmates, before most missions. You can't
buy anything because that's not the spirit of FS, you're a military pilot, so you fly what you're given

. Could be something to add in the SCP if more people would be interested with this feature though.
The ships themselves can't be customized besides the loadout, but you have an enough vast selection to not be bored and to adapt most situations.