That's difficult to say. It depends where you put it and how effectively it flows. Angle valve or parallel valve is another factor. Angle valves see cool CHT and higher oil temps. Parallel valves are the opposite. What cowl are you using?
Biggest cooler that will fit. You can always block some off or put a smaller one in later. I make a simple “throttle plate” in the exit ducting to control flow. Stewart probably has pictures
Price is similar for 9 row up to 18 row aero classic.
I doubt oil temps will be a problem. Where do you plan to locate the cooler? It should work fine up front or on the left rear baffle but the latter may raise CHTs some, and that's likely where you'll want cooling.
I'm going thru this right now. If Building using a baffle kit from Vans and Vans Oil Cooler Installation Kit, then that installation is designed for a 7 row oil cooler. The stock superb cooler is a 9 row draw cup cooler with is Areo Classic part number 8000074. The 7 row draw cup oil cooler part number is 8000075. There is enough room behind Cylinder #4 for the 9 row cooler to fit but the standard Vans oil cooler installation kit will not be "plug and play."