An alloy refurb is ~£100 a wheel, so unless you're destroying 4 wheels and tyres a year it doesn't make financial sense. Likewise PPF would potentially be worth it on a brand new car, but for something 5+ years old which most likely already has scratches and dings, it's not worth paying 15%+ of the cars' cost to have it installed. Also putting PPF on the rear is pointless. The damage will largely be concentrated on the grill and bonnet.
Also, also, aren't you always saying you have a hard budget limit of 15k? If you're looking at adding on another 2k for wheel insurance and PPF why not look at newer/lower mileage cars instead?