cam belt time

i am now having the aux belt a tensioner and an idler pully and cam belt change, now im not sure but the idler pully seems to be an extra cost that i wouldn't normally have to do? didn't i read somewhere that on an engine like this its the same part as on 240 and 740 and that they last for a long long time? its taken 2 days so far! this is my fault partly because i only bought a belt and not the kit, which i admit was both stupid and stingey! so now mech doent have the parts, the tensioner and belts are ok and i know they need doing but i dont know if i really need the idler, the other reason its taken so long is that i booked in for 9am and my local mech only started it the day after at mid-day, idiot, wont go there again but now i have, it needs to be done properly. bad mechanic tho! anyone know what else i might need?