Should I replace all lifters while replacing a few defective ones?

That's a lot of miles on the engine. All parts are worn. It doesn't seem cost effective to just replace the one or two parts when the entire engine is so worn.

I would do the full set, might as well do them all cause if you have another old one fail down the road you'll just have to do it all over again. Prevent the issue from reoccuring. Also often times, if you buy from a reputable company, aftermarket parts are as good of quality if not better because they have had time to figure out any flaws in the factory parts and fix them and half the time they are cheaper too. I do my research on anything I buy, but 9 times out of 10 I will buy aftermarket over factory replacements.