What kind of racing do you want to do? Different ATVs or different setups of the same ATV may work better depending on what you want to do.

 

Op, really going to have to give up some more details. You can race just about anything these days somewhere. Plenty of different types of racing too. Mx, xx, flat track, mud bogs, desert, ice, get the picture?

 

 

I have years of national racing experience.
There are many used 400's and 450's built from mild to wild out there. I'm a Honda guy but Suzuki's in those models are good too.
Sorry!! I have to add the YFZ450 to the list. I'd consider a YFZ for myself.

What you need to start with is a stock motor or a mid range build. Those will be the most reliable.
SUSPENSION is the most vital part of this race thing for you. You can go faster with a stock motor with suspension than a modded motor with stock shocks.
A +2 front end is what you'll want and find.
Look for a used 400/450. Remember machines that are posted as "race ready' means anything from a sticker added to wild. Study up. If you have questions. I'll help if I can.