As others have already said, there is nothing to check unless there is something wrong with the camera. The 40D is a very reliable camera. Unless you've taken 100,000 pictures with it (which I doubt), then I wouldn't worry about a thing. If your sensor is dirty and you are afraid to clean it yourself, then you may want to have it professionally cleaned, but don't let the servicing shop con you into some other kind of "preventive maintenance".
I definitely wouldn't rent a body for 3 weeks. pworm14 has a great idea -- buy a used body and then sell it after you return. It will come out a lot cheaper that way.
Have fun on your trip!
Bryan