Is Windows 8 fully compatible with latest Macbook Air 13"?
Yes it is fully compatible. But since Windows 8 is heavily dependent on multi touch gestures, and the trackpad driver of a mac isn't the best... well its not recommended. I use bootcamp with Windows 8, but since Apple are the one who need to upgrade and update both the drivers and the bootcamp for almost every major Windows realise the experince might not be as good as a "native" pc, by this I mean you can get third party drivers, as I did, and it does work, but if you buy a Mac for the hardware I suggest you look at other alternatives as it doesn't function as well with Windows as it does with Mac!

You can get look alikes that perform better on Windows! I can say personally I buy Macs because of MAC OS X the hardware is only a plus, if I wasnt going to use Mac software i would probably look in to the Samsung Series 9. However the options are many, but if your main use is to buy a Mac in order to use Windows, I personally recommend you to look elsewhere, and if you are planning on partitioning your drive and only install Windows, well honestly you can get better hardware for less money. although probably uglier! (some exceptions)

-this is from an avid mac and pc user-

If you have any questions, concerns, or need any assistance in future, please don't hesitate to reply to me. I'll be happy to assist you further.

Update 1:

Well except for the core i7 which can probably be configured in store, you can get most ultrabooks cheaper and update the ram and ssd yourself, I think my favorite pc-ultrabook (samsung series 9) is easy to upgrade yourself!
here are some links however

http://www.pcmag.com/article2/0,2817,2407200,00.asp

http://www.theverge.com/products/aspire-s7-13-3-inch/5713

http://www.theverge.com/2012/7/5/3135319/asus-zenbook-prime-ux31a-review

http://www.engadget.com/2011/11/13/lenovo-ideapad-u300s-review/

still, go in to a store and try and feel them out, focus on the trackpad! Especially in windows 8