You'll need one eventually, so yes, might be worth getting. I wouldn't be concerned about diatoms just yet. They tend to burn themselves out in time. Lots of things will grow and die back as different food sources mean different things can thrive. Eventually it will all find a balance.
Diatoms tend to feed on silicate, so could be new sand is the source.
Water changes are fine during the cycle, as long as you continue to feed the tank. The bacteria are on the rock and sand, not in the water column. So, you could safely do a water change (to bring down phosphate and silicates) and not mess us the cycle.