Fully agree with Firerat's comment on making a proper plan. It's nice to believe in Linux, but if you "talk your client into Linux" you should be quite sure that this won't backfire. If he has only one critical piece of software OR hardware that you can't get to run as expected under Linux you might end up with and angry customer... or no customer. So be careful not to push him too much into this.
Also consider the option to migrate him to open source software while he is still running xp. If he can get used to LibreOffice, Thunderbird etc. the final OS switch will run a lot more smoothly...