Dehumidifiers are fine, but you might not need one. Wait and see what happens, if by early to mid flowering you can't control the humidity then invest in one. Don't expect your first grow to be amazing anyway, you will learn so much from it and your second will be much better. I found that with my first few grows the harvest got much bigger each time.

Humidity should be kept below 40% towards the end of flowering, but if you don't have massive, dense buds then it's okay for it to be a little higher. Autoflowering strains tend to have smaller buds, so if humidity is at 50-60% and there's no mould developing then you should be all right. By the end of your grow you will be able to look back and decide what changes you should make for your second grow.