i'll go with v-spec. it's my favourite engine.
421b is ok but it kinda little hard to tune.

RB engines are expensive. i use it on some even.

if you looking for price and performance. try STS D21B (Turbo Head).
i use mine for couple of praticing and it's great. i combine it with JP-2. even beat v-spec sometimes
i believe you can get it under $200. i use it for practicing and use my best engine for racing

cheers