Advise your client like a true IT professional. 

Convince them to get a dedicated machine to run Linux from. Assembling one from reliable parts (avoid VIA & SIS chipset boards) will cost you less than 15k.

If the client insists on the MAC you can install your services on it and they'll run. But be ware, using OSX as a server opens you to a host of venerabilities... checkout http://en.wikipedia.org/wiki/Pwn2Own. It's the least secure UNIX variant.