Any older car will have its quirks. And any older car will definitely require you to have a standalone budget for servicing and maintenance (much in the same way you'd have one for fuel). Plugs in these engines are a fiddle to change, so when buying one make sure you check the history - there are plenty out there with caring owners who have it regularly serviced despite covering low mileages. If you're worried about how much a garage will charge you, why not ring round for a few quotes and quiz them about how much they'd charge you to change the plugs?