Oh, Nickie....
What a big can you have opened! We all have our personal preferences, right? Blonde, Brunette, Redhead.... (Just look at the thread recently about 'favorite wood.') Anyway, rather than push my favorite on you, I think you need to get out there and look, play and see what is going to supposedly be the Uke to end all Ukes for you. (Good luck on that one, I am trying the same thing, maybe we can form a UU support group!)
I would look at sites like Glyph Instruments, Dave Means has built some truly breathtaking instruments and wood combinations. Also Dave Talsma. He's not afraid to make a Uke from a license plate! Just those two sites alone will give you enough eye candy to think about in time to choose.
I leave you with this thought; Spruce is very much a great wood for instruments, primarily based in guitars. However, I am not fond of it in ukes. I've played a few, but it's not my bag. It's kinda like white bread. There are better choices out there. (Since you asked!) If you are going to spend the extra money on the koa/curly koa, why not go whole hog and make the top koa as well? It's gonna have a very bright sound, which you can probably affect with string selection.
Keep us posted, and I expect to see pictures, too!
Scott