The 4 off the line is the way the ANECC boys have always done it. Bear in mind it is times that count not placings otherwise it would be rallycross.
As regards the licence situation, it is a little more complicated. The SIDC card will get you on a closed to club grade event if the SIDC is a member of a club assoc or is a club invited to the event. Otherwise you need a MSA licence. For autocross you need a Nat B non-race although the clubcross mighht be different as you can run them at closed to club level I think.
Costs for autocross are the cheapest in motorsport. You do need space and a trailer though. £40 for an event £35 for a licence, £30 a tyre that last forever. £15 of gas for the event.
Best value motorsport there is!
So if someone wanted to run a scoob. you need a car, a seat and harness, a handheld extinguisher, a suit and a helmet. Some local events need high level brake lights. Strip it out and secure your battery with yellow tape for the earth and that is it. Cages are not mandatory although I would recomend it. Home made is good enough.
So a bit of work is required.