We have 14 at the moment so here goes with a little more information but I would suggest visiting some of the large breeders websites as they are a mine of information. Also take a look at the British Alpaca site which is at
http://www.bas-uk.com/ .
1. Alpacas aren't good with younger horses because the horses are too boisterous for them. They are fine with sheep.
2. They tend to make poo piles rather like stallions so the area they have will depend to some extent on your enthusiasm for poo picking.
3. 2 should be ok if they are friends but 3 would be even better. I agree if you aren't breeding then wethers would be a good choice. Top quality females are very expensive but wethers will not cost you a fortune.
4. They are very hardy and will live out all year although they would need some shelter. Bear in mind they are prey animals so may not be that keen on field shelters etc unless they have a good all round view.
5. We also use Camelibra. Sugarbeet or Alfabeet is good for improving or adding condition. Hay for around six months of the year roughly but they don't eat huge amounts.
6. Special care is very much like sheep. The treatments you use will be licensed for sheep but not alpacas as virtually nothing is licensed for alpacas.
There are effectively two types, the huacayas are the shorter coated and some claim easier to look after. The Suris have the longer coats that ripple in the wind when they run. Lots of places claim the Suris are wilder and more difficult but I was told that provided you keep their fringes cut so they can see the Suris can become just as friendly. With mine that has proved to be the case and our friendliest alpaca is a Suri.
The main issue may be finding a shearer. If you buy from someone close to you they should be able to help with this. Coloured wethers will cost more than white ones because they are less common. Always buy British Alpaca Society registered animals. The main place alpacas are advertised is
http://www.alpacafinder.co.uk/ .