Brian has a LOT of the main points listed, BUT there's a few more you really have to look at:
1) Zoning - if it's not zoned for commercial/recreational, you have to appeal to have the zoning changed. Why? So you can LEGALLY operate your business and apply for the appropriate paintball/airsoft field license (yes it does exist).
To get the proper zoning, you'll have to have things like environmental impact studies done, noise studies, an evironmental audit of the property, due diligence informing the surrounding population, etc...
2) Insurance - you really need this if this is going to be a business venture, which is why you need the zoning. With proper zoning, you can get proper insurance. A minimum of a $5 million liability needs to be carried, that works out to about $35,000 a year in insurance costs.
Now let's look at the time. You may like playing airsoft/paintball, but once you start working in this field, it will eat up ALL your time (and not pay you very well). Be prepared to STOP playing and actually run the business if you want to succeed.
I have helped set up many, many paintball/airsoft fields over the years, both here and in the States. 90% of the people that do it don't come anywhere close to being prepared to do it right, and almost all of them close within a short period of time.
The Tier One Tactical Operator
Tippmann Arms M4A1
KWA USP .45, KJW P226, KJW hi-cappa