Airport and wifi are the same thing, both based on the same 802.11 standard and they interoperate ok.
Hardware you will need to buy ...
A wifi wireless card for the pc
An airport card for the mac
A wifi enabled dsl router
Microfilters, one for each phone extension (get them with the dsl router)
There is a tool at
http://www.dslsource.co.uk/wireless_networking.asp to help you price it up (less the airport card)
That little lot will get the computers talking and give you internet access on them both.
Windows file and printer sharing is built into macos X, go into system preferences, internet and network, sharing. Never tried this myself but cow orker says it works.
Service provided are ranked/rated at
http://www.dslguide.org.uk/. I personally like zen internet, but they are a customer of the company I work for so I'm not totally impartial.
If that isn't clear or you have other questions either bang them here or mail me.