Hi,
I don;t see why you couldn't use a brush, but I reckon it'd be harder.. It took me 10 litres to do the underside of mine and the inside of the sills, and chassis etc. Only 2.5 Litres for inside the sills etc the rest was the underside.
I'd go for the spray personally, but it does get EVERYWHERE!!! Also, heat the waxoil up beofre you use it, it makes it much easier to handle and is vital if you are spraying it.