Meangreen, do you know if I can replace that lower idler pulley without having to reset the timing? My Haynes manual (I know, I know) seems to indicate that I can replace the pulley without removing the belt. It looks like I could just fasten the belt tightly with clamps (so it doesn't slip out of time) and then just replace the idler pully. What do you think?

Like you suggested, I am considering buying the entire timing belt kit. But the kit costs over five times the cost of the one idler pully that's broken ($39 from Signature Lincoln Mercury). The other pullies and the belt have no apparent wear or damage.