I'D strongly advise against this. It's too big a job and too expensive. Jaguar produced an XJC cabriolet -- a two-door convertible much like you're wanting -- so perhaps you can seek out one of those. Or go for an American convertible. The quote seems too low for a decent job. If you go ahead, find out as much as you can about the person doing the work. Also, talk to a VicRoads-licensed engineer.

BY LAW you're required to bring the car to a halt when you switch between gas and petrol, or vice versa. That's to avoid the potentially dangerous situation of losing power when switching on the road. In any case it's not advisable to be fiddling with switches while driving. Apart from that, the only other thing to do is to not have your foot on the accelerator as you switch over.