You will be fine. I've never had issues with used lifters and a new cam in a mower.
The stock steel pushrods will work just fine with your setup. You won't be able to spin it 8000 RPM with the stock valve springs, you don't want to spin it 8000 anyway. Get a set of used dual springs if you can. Get a set of billet retainers for sure. Don't run the stock retainers.
My experience with the cheeta cam is it likes 7400RPM. Any faster and your just making noise.
Paul