I think that around here if you were to try and hold someone to a fixed price you would get over charged more often than not. Fixed price means they have to charge you for "risk" and that costs a lot of money. There are so many unknowns on both sides of a good sized pond project.

The most important issue is whether you develope a trust between you and your contractor. You need to have a mutually fair situation so that he does not get hosed by surprizes he may find under ground and you don't pay "risk" dollars for problems that don't happen.

Bottom line is that you pay now or pay latter. Get referenceces, See a job they are doing now in your area, be there while they work, (not to be too nosey but to work with them on "on the fly modifications or changes" that may be needed)and trust your gut.

Good luck.