YY they are more fragile than rabbits.
Do they need to be outside? They're quite hard to manage in winter (I've had my pigs in at night from Guy Fawkes to Easter. There are many nights I've gone to bed thinking "thank goodness the pigs are in")
If you do need to keep them outside, a shed or non-car garage to put the hutch in will protect them and you can insulate with hay and covers. But it's the damp they don't cope with.
Our pigs are far too idle to use ramps and levels, they like on-the-floor-living 
Definately a pair (or more) though if you want boars, get a bonded pair (can be brothers, dad and son or two bonded together unrelated).
With boars though, 2 is the magic number
Sows can be funny little characters, they don't automatically 'get on'.
With ladies, the only limit is room, so you can have 2,3,4+.
We had boars which were lovely,affectionate,bolshy boys. You need to give them lots of room.
We had a neutered boar + 2 non-related sows (all similar age) Now we have the sows.
They're more reserved (though when we got them, my pig wouldn't even sit on my lap, so they have improved hugely)
They pee more too 