Answers:
Yes that is possible,simply a case of digging post holes concreteing them in and either using gravel base bords and slotting pre made fence pannels into the bays, alternatively for more privacy take the brick walls down and replace with blocks
Yes firstly a shingle roof would not support the weight of bricks or blocks and it would need to be on brick or block for the bond of the bed of cement
As every room is a different it's hard to give a (rough) price also depending what coving you are wanting i.e plaster or polystyrene
I have found the easiest way of doing this is to overboard the celing and skim it alternatively you can bond it over give it a coat of grit and skim
Depending what the background is i.e bonding,browning,bording or s&c yes but always better to have skirting fitted