We purchased 'moderate' quality sheets. We simply fold over/under a king sheet to cover the mattress. I agree with other comment that fitted sheets are a pain. We use top sheet and blankets in warmer weather, but carry sleeping bags too, used as a comforter. We don't want to sleep in a sleeping bag any longer (did that tent camping for years), but as an upper layer, a sleeping bag is toasty.
We don't use a mattress topper. Some people swear by those foam toppers, others swear at them. That's a personal comfort issue.
All we're trying to do is keep the mattress clean, so covering it with any quality sheet is fine by us.