It won't melt, I have some ~300mW lasers using acrylic lenses.
What diode are you planning to use? Because I'd suggest if its some eBay one, don't get it. Buy from CajunLasers or Modwerx or a forum member. That way you know exactly what diode it is.
Buy from, like I said previously, CajunLasers. I'm ignorantly assuming you're from the US though, right:? if not I'd suggest finding some members in your country using the Member Map and seeing if they have any/know of any sellers.
Make sure you do some reading on how to build a laser. It's a really rewarding endeavor and should be treated with patience.