What is the bodywork like? If it is in good condition and doesn't require much prep then I'd say around £800-£1500 depending on the paint. A lot of small one man band spray shops turn out exceptionally good work and don't charge much.
Beware of the paint selected though. I've heard of some paints alone eating up this budget.
It might be cheaper to spray one colour and get a vinyl wrap for the second accent colour.