Depending on your pc case, how much room you have etc an air cooler should allow for overclocking just fine. An h60 is a single 120mm aio and generally no different than a decent air cooler for overclocking.
I'm not sure about their warranty coverage but if you're concerned I'd contact them and ask. They're the ones who ultimately would or wouldn't warranty damaged components, they'd be able to answer better than anyone else here. The best place to ask for definitive answers is the source, our thoughts are more or less speculation since we lack the authority to issue the warranty coverage.
I've heard of places warrantying just the product, the cooler in this case. Other's I've heard warranty the damaged components, especially if it's...