You're better off finding a genuine second hand spoiler that's been carefully removed or find a complete tailgate that you can simply swap onto the car. The one you've linked is a copy therefore the fit won't be 100% more than likely as in there have been reports of them not being the right width or being too short.
As for cost of painting could be anywhere between £50 and £150 depending on the body shop or you could try yourself using spray cans which would cost £30 ish.
