I'm not a plumber, but I've done some plumbing work (nothing major like a bathroom addition) and it can be surprisingly labor-intensive and frustrating. I'd think you want to make sure your venting is really done right so you get proper drainage. You definitely want an effective flush--not for the water in the toilet just to circle around and around.
Also, I'd be concerned about water pressure. For example, plumbers know what size pipe to use and how to transition it, so they might run 3/4" to a point at which it separates into two 1/2" lines. You don't want to be scalded in the shower when someone turns on the cold water in another room.
I fully agree with not wanting to pay ripoff prices. But--code or no code--if you can find a plumber who'll work for time-and-materials it could be money well spent. If you're paying by the hour, maybe you can just hire one for a set time (say one day, or ?) and then you can take over from there. Just an idea....good luck.
[This message has been edited by k2 (edited September 20, 2003).]