My personal policy is that I don't do t-shirt quilts. I quilt for fun, and because I love working with beautiful fabrics and interesting designs. My favorite aspect of the quilt making process is the free motion quilting. T-shirt quilts are not fun to me, nor do I find the fabrics beautiful nor the design particularly interesting and free motion quilting on knit fabric, even stabliized, is just a pain. I'm glad that there are quilters out there who will make them for the people who want them, but I personally am not one of those people.
I too would suggest that you make some sample blocks and try out your technique before committing. Another thing I would suggest is that you stipulate that all the tshirts must be thoroughly clean, and that you will not work with any that are so worn as to be unstable. Be sure that you know which portions of the shirt are most meaningful to the client too-- sometimes there is a logo on front, graphic on back -- do they want both?
Finally, if you want to be paid for this project, make sure that you figure out all your costs carefully and the buyer is aware. Lots of people figure that since they are giving you all these old shirt, the quilt should be pretty much free, and they're going to be amazed to find it can get expensive.