Grow Tent/Room setup in the basement - dehumidifier or humidifier needed?

If already at 55-58% ambient RH, you will most certainly need to reduce moisture with plants growing. Exhausting outside may be enough. You'll need to monitor as the plants grow. Fully grown is releasing a lot more moisture than early vege plants, obviously. You may be able to mitigate it as tehy grow by increasing your exhaust fan, but at some point i'd wager it cannot keep up unless you got a really big exhaust fan. Does your climate remain at that simialr RH year round? Probably not, so if you grow through a 'wet' season, it increases the need for a dehum even more than what you are measuring right now. If you keep measuring as the plants grow, you'll have some time to go buy a dehum, if you want to hold off... You'll see the peak RH start to slowly rise as the plants grow.. or not if the exhaust can keep up. It won't be crazy high day 1. it'll take time to rise and you can make a common sense prediction based on size of plants at that time as far as how beefy of a dehum you need. I strongly suggest going bigger than what you think you need. If it has to run continuously (autodefrost cycle - on cycle nonstop), it'll die in a few years.