its only like 7 or 8 quid for a big can if i remember correctly, any DIY shop sells the stuff for about a tenner a can
yea just sand everything down and start again but take your time and build up the layers slowly, good tip is keep your hand moving so you dont spray too much in one place
again halfords lacquer does the job
also sanding and primer first is a good idea because it gives better surface for the paint to stick too, if you are spraying straight onto the plastic without sanding or priming first it wont work too well