When my dad was looking for a new wireless mouse the salesman said that most mice need those dongles because they don’t actually operate over Bluetooth but things like infrared because Bluetooth is too slow or unreliable. If this is true than I can see why MS didn’t use Bluetooth for their controllers.
I don’t know much about the viability of doing it your way but I have a few points I’d like to share.
If you come out with a cloud synced app for only one platform then you are limiting its use severely as it really just becomes a backup service as most people would only own one W8 device so get it up and running on the other ecosystems ASAP.
Is the main goal of this app mostly experimentation and getting your name out there/building your portfolio? If so keep the price low.
Also out of curiosity what sign in service are you going to use for the Windows to Android sync e.g. Facebook, Google, MS account?
Anyway you can always change pricing later on if you find things aren’t working out the way you want them to.
There is already built in syncing for small amounts of data designed for keeping content in sync. Example here. It currently only works with W8 but will probably be in WPBlue. MS wants people to subscribe to Azure for larger data sets.
With the update on the one of the APIs that gave you the option to download an app when you sign into a website with your G+ account they could easily expand that to link between chrome and android and vice versa so there is an easy way to keep all your apps in sync.