You don't need an IA an A&P might do especially if they are familiar with RV's. As Mike S. said an experienced RV builder owner would be better than nothing.
There is no requirement to hire anyone for pre-purchase, but you still take the lead regardless of who might hire. You should be there. There is no guarantee you will get a great plane even if you hire someone. You should be proactive and present during the inspections and of course fly it.
Paper work: Have a good look at logs, registration, inspections, condition inspection, pitot static transponder, ELT battery. How well was it maintained? How much did it fly? If it's the builder you should see their build logs. So Cal tIere are many RV fliers. Are you an EAA member? If not you should be. You can find many RV and experienced technical folks to inspect your potential plane.
Condition: You have eyes, functional check of everything. Avionics are one of the most expensive items next to engine.
Engine: Regardless of airplane the engine and prop are the most expensive items. A compression test is in order if you don't trust their numbers they recorded last condition check. With cowl off and flash light look at everything.
Take inspection covers and wheel pants off... is there anything to gain? You will see if they primed inside and general qualuty. Sitting on flat ramp get back away from plane and 360 it. Does it sit wings level? With wheel pants off you can see condition of break pads. The owner may balk at removing covers and wheel pants.... It is a pain and you risk scratches.
Flight Test...... this might be an issue with RV3 or RV4 or RV8 with solo controls, letting you or your representative fly the plane. However you want to do some basic performance checks. Does it trim up? Heavy wing? What is the WOT max speed at low altitude? What do the engine instruments read. If fixed prop what is static RPM on T/O climb, cruise and max speed run.