Just pay someone to do it, much easier than arsing about yourself and screwing it up. You want it to be perfect, not put it on and wish you had got a pro to do it, because everytime you look at it you think... thats not quite right somehow!!
Id actually probably say the pre-coloured one might be a good idea, but then you may find your car's colour has changed very slightly over time, and it isnt 100% match; so you may find it better to get someone to colour-match generic bumper instead.
Where is the pre-coloured one from, Xpart? If so, they may be able to get one for you to look at before you buy. If not, id probably be hesitant to buy a pre-coloured one, especially over the net.
I wouldnt have thought getting it painted would be more than £70 or so, so even then it would only be the same price as the pre-coloured one.