Popping in the exhaust is normal on the overrun, it's the AIS emission control doing its stuff. If its been standing you'll need to give it a full service, so factor that in the price. As for the "old fuel" in the system; if it runs, ticks over evenly and pulls OK it'll probably be fine, but only time will tell. Drain what fuel is in there and get some fresh stuff sucked through ASAP and it'll clean itself out. (FI systems aren't quite as problematic as carbs regarding old fuel gumming things up.)

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