While it may be no big deal to get the printer part of an MFP working in Linux, the scanner part is another question.
I have a very cheap Lexmark X1270, which was essentially free and works with Windows XP on my Compaq Sempron desktop computer. I didn't expect much from Linux. To my surprise, I found that the inkjet printer part works quite well through the usual kludgey z600cups driver installation routine. The scanner is a lost cause, however.
There is a Lexmark MFP scanner driver for Linux available on the Web but it is not for this model and it doesn't work with it. It's frustrating because SANE sees the scanner and identifies it properly, but scanning doesn't work unless something has changed since I gave this a pretty intense try a few months ago.
The copy function doesn't work, either, because although you can copy just by pressing a button on the device, copying depends on having a functioning scanner and also the Lexmark software for the MFP, which is Windows only.
I have an old Microtek SCSI scanner attached to another computer and that scanner is very well supported in Linux. So it's not at all crucial for me that the Lexmark scanner works in Linux. It would be nice, though, because if I want a quick and dirty scan of something, I either have to turn on the other computer (which is usually off) or reboot into XP, do my scan and save it on a drive I mount in Linux, and boot back into Linux. With VL 5.8 Standard booting in about 25 seconds, this rebooting stuff is not particularly taxing, but nevertheless, being able to use the scanner in Linux would be nice.
So assuming the client is interested in an MFP because the client wants a scanner as well as a printer, I'd advise checking out whatever you're considering to make sure it works well in Linux. There is a table athttp://www.sane-project.org/sane-mfgs.html#SCANNERS
but I'm not sure how up-to-date it is.