I'll let someone else answer your main questions, as I haven't played properly in quite a while
But, welcome to the forums!

UO CAN have a more mature/tightnit community, but it can also have some of the worst elements of an online community
Which is why guilds are such a good idea
If you want my totally biased opinion , pick Lake austin as a starting shard and join the UOForums guild
They're a great bunch of friendly/helpful players.