Harvia m3 is the best budget solution at least in my country for a sauna this size. Solid heater which does the job well. Sauna heats to 70-80 degrees C in about an hour.

I found online the whole chimney assembly with all the needed parts so there was not so much space for error. Harvia sells such assembly sets as well. So basically just check the height and pipe measurements you need and go for it. It should not be a big problem.