I don't know where you are, but note that if you buy it from another country then it may not have any warranty with it. I wouldn't do it, but it's up to you.
If you buy from an Apple store (didn't know they had them in HK though), then you can have it installed for you yes. Else you can buy it on the online store (assuming they have one in your country), or from any other mac retailer.
Or, just try out downloading and using neooffice: it's really just openoffice for the mac, and works very well as a microsoft office replacement (about 95% compatible, and you probably only use 20% of the features anyway).
http://download.neooffice.org/neojava/en/index .php
It'll only be beneficial for you to install Windows XP if you plan on using it. I wouldn't though, unless you have to for your line of work. It'll open you up to lots of security issues, and be a pain to use anyway unless you use Parallels (virtualisation).