second bike purchase concerns

I don't know if it's the bike model so much that matters as far as cadence is concerned, but the gears you're using. Your road bike should have a range of gears that allows you to spin at whatever cadence you're comfortable. If not, you can pretty easily switch out the cassette for something else.

An ill-fitting bike can cause all kinds of pain, so regardless of the model you might have to fine-tune some things. I had knee pain on the bike and some slight adjustments to my cleat position helped. But if you're getting knee pain when running, maybe it's not due to your bike, but your running shoes or something? I didn't quite understand that part of your question.