91 Subaru (catalytic converter?)

Hi! I have a 91 Subaru Legacy sw (4wd) 180K, that I just bought. It seemed to run really smooth for about a week, and then it started making a rhythmic sputtering sound from the hood end of the car. This would only happen at about 25mph and over and only when I was pressing on the gas. The sound was very quiet, I thought maybe it just needed an oil change/fluid check, better gas than the cheepest(it had been sitting for five months before I bought it I found out later) but after I did all those things the sound just got louder and louder and I began losing power (car takes very long time to get up to speed)
Is this something that will hurt my car if I don't get it fixed immediately(like today) or can I shop around for an auto mechanic for a week. Autozone told me it sounds like either the cataliytic converter or the exhaust donut(or both?) Does that sound right? Also I've heard the cat. Converter for subaru can be fit with a universal part. Am I better off getting the subaru one? One last thing, what is a fair price for a repair on a cat. Converter or a replacement (autozone said $91 for the part, but I'm sure the labor will be a lot more, and I don't have much to spend) Sorry this is so many questions, I'm really grateful for this site! Thanks so much!