I lost track of wear on my Terry's chain and it is very, very worn. I assume the cassette is very worn as well, I have never replaced it. I bought the bike used and maybe put 6000 miles on it. How long can I go along this way before I harm my chain rings on my crank? How will I know if I have worn out those as well?

This bike is a steel bike and a nine speed on the rear with a triple on the front. Does it matter what level of cassette to get? Does going cheap matter? I use this bike like a hybrid, riding it around towns, geting groceries, etc. and it has has on a rack exposed to weather as I drive all over the USA.