I don't think having 30 paying users is a realistic expectation and I wouldn't be comfortable entering my key onto an unknown server.
There must be a cheaper host you can use as Blockland isn't a demanding game; something cheap enough that you can support it with just 5 to 10 paying users.
Make people upload their key.dat.
I'm not certain, but I believe the key.dat is tied into the hardware you are using somehow. This means you can't simply copy the key.dat file across to different machines.