I went for the first set of £14 ones on ebay. Chose the closest colour to my car. They're close enough for now. I have a few scratches to sort, so might just colour match the sensors too.
As for drilling, I was fitting a replacement bumper anyway, so I could see (and use) the existing marks to drill through (Ford* very kindly marked up the bumper for sensor holes on all of their Focus range - well, certainly the mk2.5) with it off the car.
If you're doing it with the bumper on the car, I'd suggest using a strip of masking tape across the full width at the height you want the sensors. It's easier to mark on...and if you can get rough measurements from a factory fit OEM setup, you can make it look almost stock
You will get a hole cutter with the kit, but a small pilot hole is a good idea. And run the blade of a sharp knife around the inside of the hole before pushing the sensors in.
(*Yes, I know that a Ford Focus isn't the same as a Fiat Bravo, but the premise is the same...holes in bumper, wires into car, tap reverse light for power and hey presto)