70 acres was probably drawn from an previous example Brian investigated....simply adjust it accordingly.
One aspect to consider is the variety of the venue. Static venues (indoor) quickly become overplayed and, super awesome setup or not, become boring. It's like playing the same level of a game, over and over and over. After 3 rounds on the same indoor warehouse I'm bored...but a 2 week break from it will refresh it.
A varied outdoor field/forest area is "naturally organic" and if it's large enough seems infinitely varying.
Having more areas and limiting play to one area now and another later keeps things going.
So don't discount building in the costs of reconfiguring the indoor venue a couple of times into budget...or coming up with a system where it's flexible any/every time.
There are training setups with "mobile" walls that can be rebuilt like office furniture. Some are shoot through proof and can be used for live fire training....expensive though.
There's a setup here in TO that's built to be reconfigurable...but the indoor venue is tight for the size of the buildings that can be moved around...so really all that's possible is turning a small 1 room building around or shifting it over a couple of feet.