If you have a 42-day-long render and compression session scheduled, how about charging for about two weeks to a month's worth of work? It probably won't amount to more than three days total of actual hands-on work, but you'd need to look after the computer(s) to be sure the power is up and there are no hard drive crashes or kernel panics.
The most obvious thing to do is get an 8-core Mac Pro, cut the rendering and compressing times by a week or so, make the client really happy, give yourself a time cushion in case there are technical difficulties, make a good amount of money and have a sweet system at the end. Win six times.
If you're talking specifically about using Compressor for the compressing jobs, you'd want to figure out how to set up a network for Qmaster distributed computing. That would be well worth your time. The how-to is in the PDF that opens up in "Distributed Processing Setup," which is in Compressor's Help menu.