Standard plugs for RD400s and later 250s are NGK B8ES
Early 250s and 350s use B8HS short reach plugs. My bike had B6ES plugs in it when I bought it, presumably to burn off excess oil. No wonder RDs gained a reputation for holing pistons! With plugs rated too hot, and spotty nerds not keeping timing in trim... I now run B9ES plugs as I have skimmed heads and a mild stage 1 tune so I need to run the cooler plugs. Only go up to B7ES if you have severe oiling problems. You run the risk of holing pistons or detonation by running the hotter plug.
Use an 'R' plug (eg BR8ES) if you don't have resistor plug caps. NGK plug caps (the red plastic ones) are usually resistor type. The resistor is only there for RF suppression, so don't worry too much if you don't have one, although it is advisable to fit them eventually. It works out cheaper to get the resistor plug caps and non-resistor plugs.