You're thinking of THINWALL, this carries much much more current size for size than the older stuff, and is more abrasion resistant too.
Depends entirely on which circuits you need to power, but the load capacity will be on the sites, for small orders www.vehiclewiringproducts.co.uk are very good, but www.polevolt.co.uk is another and www.automarine.co.uk
Sensors need only a tiny supply(current wise) so shouldn't need any more than 16/020 for these, and most stuff on a engine loom.
tabetha