i can't comment as i've never owned a Mk5, i think the A3 looks better, but i think the Mk5 has a better sat nav and trip computer, so it's swings and round abouts. but i don't really see the point in swapping from one to the other. plus Golfs do have some issues with rust which the A3 doesn't have.
shouldn't cost £1.5K to get Xenons refitted - they go on eBay for about £500, and you don't strictly need headlamp levellers or washers. if you were to go for them you don't need a new bumper, you can cut out holes from your current bumper