£120ish for a wiring loom I think, and then some labour to fit. I have my old one but it may well be damaged, I don't know - not worth the risk at that price is it?
The coil packs might show corrosion so yeah, just a visual inspection.
You won't get a visual inspection on injectors though, they're too small - mine were sent away to be tested and refurbished, I guess ultrasonically cleaned. Two of the four were screwed and had to be replaced, the other two were refurbished.
However anything involving injectors will be expensive because of their location - you could be talking £500 in labour to dismantle and replace. It's worth finding out exactly what was done previously so that you don't waste money repeating it.
In my case I started getting misfires when accelerating at slightly low revs, e.g. block change 2nd to 4th and give it throttle and it'd happen. I think this was briefly resolved by changing a coil pack & spark plugs, but the injectors started giving me grief immediately afterwards - eventually couldn't hold idle. I was convinced that the latter was an electrical problem but it turned out to be injectors.
I'd do this, in order:
(1) DIY: visually inspect the coil packs and the wiring to them, and you may even be able to test the resistance with a multimeter, not sure about that
(2) Possible DIY: visually inspect the spark plugs and make sure you have the right ones for the JTS (you probably do)
(3) DIY: inspect and use contact cleaner on engine bay connectors, namely:
(a) the block connectors at the back of the engine
(b) the coil pack connectors
(c) the lambda connectors - four of these at the front of the engine bay
(4) DIY: use injector cleaner in the fuel tank - Forte is meant to be good - and run on super petrol for a bit
(5) have the wiring loom replaced
(6) get your injectors tested and refurbished
The worst case scenario is that, like me, you reach the last stage and the whole affair has cost you about £1000, but I'd be very surprised if you don't end up with a fixed car.
Another question to ask: what's your fuel economy? (measured at the pump, not from the computer)