Re: wall is suporting... what next
We've just gone through this recently. You'll certainly need a horizontal supporting beam. Depending on the design of the remaining walls at either end, you may need 1 or 2 vertical supporting beams to support the horizontal supporting beam. You should get some input from an architect or a structural engineer to be sure. Decide whether you are happy to have the beam 'showing' at the ceiling, or do you want to pay an extra few thousand to have it 'hidden' in the ceiling space. Find out if there are any pipes running through the internal wall (any radiators fitted?) which will have to be diverted.
I can't give a specific cost for this, as it was part of a larger job we were getting done, so I don't have a breakdown of costs.
I'd recommend a reputable, established builder, though you can expect to pay a premium of 20%-30% for this. I'm amazed to see neighbours carrying out major structural work like this via a 'man with van' type builder. You should get a completion letter from the building confirming that all building regs have been met.