You are going to need a bypass/unloader valve (this is the valve the shunts the flow of water from the pump's output back to its input when you release the trigger on the nozzle handle). That can run between $150 - 250 right there. I have a Graco pressure washer that has a pump very similar to what you just purchased. It is powered by a 18HP Briggs Vanguard V-twin engine. It's a beast, but $$$$$$ (see below).
You need to look up the technical specifications for your pump to determine what HP it needs at what pump speed. You can back off the pressure of any pump (around 2K PSI is typically sufficient for most cleaning applications) which will reduce the HP needed somewhat.
My hunch is that you might be able to run that pump with your 9HP motor if you run the pump slower than its maximum ratings, and run at a lower pressure as I stated above. You will then have to determine what your output flow is in order to select the proper nozzle size.
I'm with the others here - I'd just scour CL for a good deal, after you have spent more time online researching good pumps. Do you have any professional pressure washer sales/service dealers in your area? That would be an excellent place to start. They can teach you a lot about what is out there and what would be best-suited for your needs.
Even having the motor and (unknown-condition) pump already, you're looking at probably at least $600 worth of additional items (a few hundred more if you have to go through your existing pump) to get a functioning unit, assuming that you know how to put everything together. Shoot, you can spend $150 just on the belts and pulleys!
A couple of years ago, I had to replace the unloader valve on mine ($250 just for that one part). After walking out the door of the local place, I had a bill of over $600 for everything else (parts, accessories, turbo nozzle, etc) and I already owned the pressure washer, hose, and gun! A high-quality pressure washer is not inexpensive.
Good luck!
