IMO.....If you are going to try to go out and 3D grade as an owner operator you need not fool with your GCS900 system. The radios suck, swap out your 900MHZ radios for some spread spectrum radios....you will have to get a license, it is what the surveyers use. You can use a GCS system but I advise against it for tight work. You could go with a robot, but that is now really dated technology. However, IF I were to go back to robot grade I would use a good reliable ATS600 setup. Very stable gun compared to the new SPS930 guns. Not as fast but better IMO. I have had dual GCS and they do not work half as good as the MC2 by Topcon. It really sounds like If I had (I have) your setup, I would invest in an ATS 600 and a prism IF you already have a Trimble setup on your blade. If not, consider using the parts from your SCS 900 to put together a dozer??
Get a Topcon base and 3d-mc2 system for your blade. Use you rover on the topcon base. Or trade it in or sell it and put it toward a Topcon purchase. The Topcon software is a lot easier to model yourself than trying to model under Terramodel also. You can cut .01 grade all day long with a tight machine. Very accurate GPS system. The trick to making clean grade with 3D is all in the model and RED HOT control. Learn to build your own or you will not make it in 3D.
Something you can make good money at is grading 3D between the curb. No one wants to do it though because it is between the curb and curb sucks! Except that burning smell when you are giving it a rub.....let me get back on track.... It can be time consuming and probably only worth your time if you are an owner operator building your own models. Take your ATS600 if you are going with Trimble or your Topcon rover if you decide to go big or go home, before you go to the job, set up control, and topo the curb in 2 to 5 foot intervals. Closer through the grade breaks and vertical curves. Offset your grades on the model if there is an AC roll over the curb to the thickness (.02) around here, install your crown by average, pop the model into your blade with the control file and blow and go. If you have all the bugs worked out you should be able to finish about 12-15k sq yds a day. You are probably better off just bidding to the developers to do the finish work with your blade a 9 wheel, water truck and a 613. The freehanders will not be able to get 50% your production NOR match you if you make the system work for you. My friend is by far the BEST I have seen with 3D with all the ins and outs. He is at $167 an hour with a model operated, job provides the fuel and cutting edges. However, he in many minds is the BEST. I can manage $150 hour. Good Luck.
