(For starters, you won't be hearing from Bob on this Forum ...)
I suggest you start 'defining' your dream on paper. A spiral staircase may limit what you can 'move' or carry into the attic. They're nice space-concervers but they're also dangerous too.
Some things (among others) to consider are as follows:
PROPER ridge AND soffit ventilation. Proper support on the floor since an attic is 'originally built' as an UNfinished space. Proper load on the HVAC system since you'll be heating and cooling more space. I'd forget plumbing up there since it will definitely involve more demolition than you want in the room(s) below. Complete wiring of the new space for phone, cable, and electrical.
An attic is an interesting space. There's less to work with than you think when it comes to making it a habitable space. Building Codes are very tight! Do realize that in case you need an emergency exit, an attic is a death trap unless you design a way out (or better yet, 2 ways out!) IMO, you may want to consider building an addition or even moving. A finished attic doesn't really add that much value to a home, and IMO, takes WAAAAAY too long to recoop the costs involved.
There are PLENTY of books at Amazon.com and at BarnesAndNobles.com and at Home Centers that you can buy to help give you ideas. I suggest you spend just a FEW $$$ going this route before you go hiring an Architect (which I suggest you do) and a Contractor. There's a LOT to consider. You certainly don't have to 'believe' me but I know a little about this and (with all due respect), you don't know much yourself.