The shocks will be tired. Fitting lowering springs will put them under more stress. If fitting springs you may aswell replace the shocks too. Cheaper than coilovers.
I made the same offer matey, one of his mates put it in his head that they were fetching £5k up lol. And it does look a clean car, be good if he doesnt drop a silly reserve on the car.
I paid £3400 for my advance with 37k on as a fresh import. That car is alot higher mileage and not in the best of condition. Unless the car is gonna stand out, it wont get much interest.
Unless you're running stupidly high boost levels or have an issue where it's blowing the spark out, not really. And depending on the ecu, its possible to retrofit at a late date.
I'd imagine halfords and local auto factors may stock them. Otherwise you can just get them off ebay, keep away from the alloy/aluminium tuner nuts though lol.