Solo while doable is quite difficult, i've played solo a lot and find it quite fun and challenging.

And yes, even if you play solo you can later jump in with others you find. i suggest looking at the "looking for group" forum when you decide you want to.

The character you create can be used in SP and MP and the gear you have will stay on you too. So remember that if say you want to save that particular gear for your SP game and don't wanna lose it.