There is no real reason why a decorator wouldn't do whatever you require, but most will want to do a job to completion. That way, they can't be blamed for the end result if it turns out badly. Most would prefer for you to strip the walls and then allow them to prep, paper and paint (I know that I would). The problem there is that you may cause more damage to the surface when stripping than a pro would, resulting in preparation time/costs being higher.
It's wise to get more than one quote - some will overcharge so you really need to compare several prices (at least three).
Try to factor in how much they would want to earn per day to get a rough idea of an average price - materials supplied would be additional. Allow 1 day to strip, 1 to prep very bad surfaces, 1 to paper, and 1 to paint, i.e., 4 days max. for an average sized room, depending on the prep required. If you get a national average charging decorator, you can expect to pay approx £125 per day. Therefore, each room should cost around £500 from start to finish, again depending on size and the amount of prep involved.
Bear in mind that, for a good job, up to 70% of labour could be in the prep work - the actual painting should usually take the shortest amount time - so, in theory, doing that part yourself wouldn't really save you an awful lot of money.