I would FIRST visit and look at other saunas.... !!! What facilities do they have !!! and are they WELL Maintained??, and their opening hours.
Remember where there is WATER - much maintenance and cleaning to be done .
The golden rule it MUST be CLEAN.. always, and ALL the key elements working always !!!
Elements:- suggest Sauna, Steam Room. Communal Showers ( where each of the jets come on at the same force at the same time !!) Jacuzzi.
Lounge area- Tea / Coffee. cold drinks anything more WILL require some legislitive health & food standards... that will cost!!! keep it simple !!!
Guys like some form of private rest areas.
Hope this helps....... As regards Liverpool- the city one closed maybe highish rent and few clientle.... so take note !!!!and it was a huge space.
The one on the Wirral whilst small and compact and generally ok ish !!, has a few shortcomings... ( jacuzzi no jets, and showers very iffy )
Hope this helps ... i am not in business, but running one as described above will take some investment, and maintenance.
I wish you well if your dream comes on stream. Not seen a new sauna opened for a long time .
regards ...Sea Blue