[fidspeed]

Lots of great opinions here

For my tuppence astro outdoors with sand can be a set of tyres a run at regional level not good for club racing
I dislike sand in cars any way
Carpet is a better option
I really like worksop rubberised floor as you can run tyres till bald disadvantage is grip levels increase during day
Clay may require old school cars which are no linger popular and clay may be difficult to maintain

Big thumbs up is getting opinions first before ploughing in blindly
Can you divulge where exactly the venue may be that may give you a better idea of likely area of draw of drivers/customers

Look at others failures or success to help you determine why they failed and help you avoid similar pitfalls
Good luck Dave