Hi Bob, additive fabrication is probably not what you want. If you are using O-rings you are trying to seal something. 3d printing is not known for it's air/water tightness. Also as you fear, the side of the part with the suspended area will need to be printed on supports.. this is where the slicer creates little towers under the suspended area so the first layer of the suspended part has something to land on.. this leaves a very uneven surface..
I would highly advise against the purchase of a 3d printer for this venture as it is not the proper tool for the job.
Please remember I am making assumption as you did not provide details on the parts design or what it will be doining.
Good luck!