Plus you will only have to go through the hassle of doing it once.
Same goes for the guest house, the structure is pretty inexpensive, it's the finishes that add up. Will you want a full kitchen, kitchenette or will you be ok with a mini-fridge and a microwave? Will you want ceramic tile floors and bath surrounds or will you prefer stone or glass tile? On the simple end, you're probably looking at $100-125/SF and it will go north of that the more you intend to do on the inside.
I agree that it will be more economical to combine the the garage with the guest house. Otherwise, you will have to run multiple electrical lines from your main panel to the varied location which will result in added cost. That cost will really increase if you choose to add plumbing to your garage.
You may just want to get in touch with a general contractor. They will ask you many of these questions to get a better sense of your project to give you realistic figures. Unless you find a GC that has CAD design skills, you may be looking at using a draftsman or architect for the design/permit process to be able to submit your plans to the city, so add at least a few thousand more for that, as well. I can gladly give you a few GC referrals for your area if you give me an approximate location in Scottsdale.
Good luck to you! It sounds like this project will make your home a very enjoyable place to live!
Realty Executives- Paradise Valley