To do a GOOD job requires the car to be stripped, ie all windows out , wheels off , door trims off and if you are doing a full colour change properly then engine out and engine bay stripped.
If they aren't charging 6K for that , then they are too cheap and you get what you pay for.
Colour changing a car is a good way to spend a lot of money devaluing the car.
When the time comes to sell , most people prefer the unmolested , unsprayed original colour. Colour change to me denotes something slightly amiss and I price accordingly.
Get a quality 3 stage type polish , Mothers is my personal favourite but Mequiars is good.
Allow about 6 hrs and a lot of effort and follow the instructions ( ie. not in the sun , small amount at a time , allow to dry etc ) using all three stages including finishing with the carnuaba wax.
The results are usually stunning.
Cheers
Ken