You don’t want it to drain below the cutoff voltage, and since batteries are usually shipped empty you should definitely charge it. Probably healthier to keep the battery at 80 or 70% charged when it’s just sitting unused in terms of battery lifetime, but that’ll get decreased way more when you finally get to use it so I don’t think it matters really.
batteries don´t come empty but in storage voltage, which is 3,4-3,6 V on Liion cells. As that´s a prebuild pack that can be different. If there is no bms or anti spark connected than it shouldn´t be an issue. If it is, than there is a constand energy draw from your pack. very less, but over some month it could drain your pack too much.
I would check the overall voltage now with a multimeter and repeat that all 2-3weeks. As long as the voltage of your pack is over 30V all is good. Ideal would be 34-36V for sure.
If your pack is now charged up to 42V that´s also no big hassle. I non the less would check the voltage from time to time to make sure it´s not draining too much.
a good moment to get one. It doesn´t need to be the 200€ Fluke multimeter. Just get one for 20-30€ in your electric shop around the corner. Believe me, if you work with batteries you want to have a multimeter. You will need it multiple times later during building.