I had apple care protection plan - managed to get it free as part of a promotion when I got my macbook pro (about 4 years ago) - it basically allows you to take it instore or send it off for repairs if some breaks in it. Mine managed to slow down 3 year and 4 days after I got it and my protection had ran out - but I was able to just sort it out myself as it was overheating as well. If you're the negotiating type could you not try and get a discount on it?
I've used an imac in work for a couple of years and not had any issues with it - Apple's products are built well so I'd only really consider taking APP out if you can afford it and want to be extra precautious. Might be worth taking out insurance on the house/building you keep it in as that will cover theft - APP doesn't cover this.