If I remember correctly, your Sea Ray 300DB has Morse/Teleflex cable (push-pull) steering with a tilt steering wheel. The steering cable connects directly to the rudder arms. If so, you can probably use either a Comnav 1420 or Si-Tex SP-80, both of which are available for cable-drive steering. These autopilots replace your existing steering drive assembly (behind the dash) with an integrated autopilot drive unit that connects directly to the steering cable. They have a separate control head or remote. I'm not sure if they are available with a tilt wheel, so you may have to go with a fixed steering wheel angle. I would definitely check with the manufacturers for recommendations on your specific application.
Raymarine makes a 'Sport Pilot' that bolts directly to your existing steering system. They are generally used on smaller boats, but if your steering load is light enough, this could be an option.
I doubt that a belt-driven 'wheel pilot' often found on sailboats would work because of the steering wheel size and angle in your SeaRay.
From experience in a prior boat, I can tell you that one of the most important aspects of this installation is to make sure that your existing cable steering is in PERFECT condition with MINIMAL slack and friction. You should be able to turn the wheel with one finger. This would be a good time to install a new, high-quality steering cable.
Beyond that, I would check boating publications and boating forums for more information on autopilots for cable-steered boats. Specifically, try to find feedback on this type of autopilot installed in boats similar in size and hull form to your own.
If your boat happens to have hydraulic steering, you have many more choices, Simrad, Furuno, Raymarine, Garmin, Comnav, Si-Tex, and on and on.
My advice is to take your time, do your research and install it properly. I mostly single-hand my boat now and love the freedom afforded by a solid, reliable autopilot.
Good luck.