Ill post, first, you would be a heck of a person to change tires off the rim yourself. Its possible and people used to do it, but I see no value in that unless its a lawnmower of four wheeler tire. Then you would also need to balance the tires yourself, or they would vibrate your car to pieces, so on and so forth.... Now the next item.
Ive never used costco, but price is king. With that in mind, NOT ALL TIRES ARE CREATED EQUAL. I personally only use Michelins when possible. I swear by them. Ive never had a blow out, flat or anything, maybe nails are scared of michelins? IDK. Other people will say they are too expensive, too each their own.
With that in mind, My understanding is that your car is old. If you think you may send it to scrap yard in 20k miles, then a top shelf tire really doesn't make since, but If you can squeeze another 50k miles out of it, I would splurge on good tires. Good gripping, quite, smooth, well balanced tires can really make a difference on an old car. Does to me anyway.
Im almost certain everyone else will recommend getting whatever is on sale that week. My wife works for Michelin so maybe thats another reason I like them so much?