Airfix 1/32 scale kits are quite good for slot modifying - there must have been a Commer one made at some point - I'll go check ebay.
As for the venue, I have to say that I have the ultimate Scalextric track designed (but not built yet - got all the track but needs to be outdoors and no good weather yet). Just under 13 metres (tad over 40 feet) x 5.5 metres (18 feet). Will dig out the design.....