It all depends on the rpm you plan to run. Negative on the piston, negative on the valvetrain IF you use 18LB valve springs. Any spring heavier than 18LB really should have 3/16" pushrods, stainless valves with automotive retainers, and possibly an upgraded rocker arm. People can argue the cutoff for when to use each component but that is my conservative opinion.
Really a flywheel is a good idea so you don't shoot 6000rpm cast iron into your leg at mach 1.7. This is especially true if you are using a Predator or other China engine as their flywheels are of lesser quality. Depending on what engine you have there are options as cheap as $45.
Also, a billet conrod is required for rpms above 5500rpm. The better handle on oil level, quality, and maintenance, the longer it will last.