Considering an extension - have some questions

We're considering building an extension on our house to make a dining/living room area. The house is a timber framed 3 storey mid-terrace townhouse with a brick skin. I would like the extension to be the full width of the house, which is a little over 5.5 metres, and project 3 (or slightly more) metres out, and also using timber frame construction. Ideally it would have doors (sliding or bifold) across most of the width, and the internal ceiling would be open to the roof with roof lights. I'm trying to figure out how certain things would work, as I'd like to know what is possible before getting too carried away with ideas or wasting anyone's time!

The back of the house on the ground floor is a kitchen diner, with french doors off of the dining area and a window in the kitchen area. We would not be planning to reconfigure the kitchen so would not need to make any structural changes to the back of the house. I was thinking we would remove the french doors, and leave the window in place. However, if we did want to be able to open it up in the future would it be possible to retrofit the necessary support or is this something we should look at putting in place when building any extension? Is it even possible to open up a 3 storey house right across the back or would a central support always be required?

The boiler (conventional not combi, if it makes a difference) is sited on the external wall in between the french doors and the window. 3 fan vents are also on this wall - extractors for the kitchen, utility room and WC. Can these vents and the boiler flue be redirected up and out through the roof of a new extension?

There is an external tap that is fed from beneath the kitchen sink under the window. I'm assuming that a new tap could be sited on the external wall of an extension and connected to the water supply easily enough?

There is a down pipe for the guttering on one side of the house, which is connected to guttering both on our house and the neighbouring house. Would this pipe (and the drain it flows in to) prevent us from building as close to the boundary as would otherwise be possible, or is there some standard way to deal with this sort of issue?

Finally, the piece of string question. Any rough figures on how much this might cost in central Scotland? We would need basic electrics and heating plus dealing with the issues I've already asked about, but other than that just an empty space. I'm anticipating that a lot of the cost could depend on the type of doors we would want, as I believe bifolds etc. are rather pricey.

Sorry if these are silly questions or impossible to answer - we've never had any major building work done and my parents haven't had any done in a very long time or in similar circumstances. I'm happy to provide more information.

Thanks in advance.