Best Core to start with?

All C5's have lock-up converter and you don't want to race them. The lockup clutches will burn up spreading debris through your transmission - that is why you change them over to the stronger C4 converter and with the change in converter you use the C4 valve body because you don't need the lock-up part of the C5 valve body. You will need to use a filter with a deeper pick-up or get an extension for the filter.
Paul

I never intended to use the C5 converter, it's in the trash unless someone wants it.

For a valve body I plan on a PA reverse manual unit.
[addsig]

I have to work at remembering that this is for autocross use.
A BIG cooler is a good idea, and an in-line transmission filter too. You can change it a lot faster than you can the filter in the pan and you will want to change it after every race to start with. AS you go along you can plan the services better. Use type F fluid in it - it is less temperature sensitive and allows the clutches and bands to grab a bit quicker. Be sure to adjust the bands per the specs that come with the valve body. You might want to install a temperature gauge in the case or pan to monitor the oil temp to make sure it is running cool enough when you are rowing through the gears.
Paul