I've been involved in a dozen or more building/renovation projects and its always cheaper to renovate than build from the ground up. I've see this both in projects under $100,000 and well into the millions of dollars. Sure its always nice to have everything brand new but there is also a price tag for brand new.
Not consider the cost of building VS remodeling I would think that as a theater group you really don't want the monthly liability of owning a building. I would estimate that your monthly overhead for a building that seats 200 - 400 people would run about $4000 - $8000 including utilities and insurance. I would think that leasing a space would be far cheaper on a monthly basis as well. Probably in the ball park of $3000 - $6000. Depending on your revenue per show and number of shows you do each year having a fixed expense of $36,000 - $72,000 a year may even be a bigger killer than the high school charging you random fees per show.
My recommendation would be this. Negotiate exclusive rights for the movie theater for 5 years. Renovate the theater is 2 phase. The first phase do basically the bare minimum to open the theater and get as many volunteers and business sponsors as possible. You'll probably still need to spend $30,000 - $60,000 depending on how many systems issues you have. (Electrical, AC/heating, plumbing, etc) The second phase I would start in year 3. Finish what you didn't / couldn't do the first time. Probably spending another $50,000.
As far as the monthly lease goes on the space I would try to negotiate that for 5 years you would only cover the utilities, insurance and that you would split the cost of building maintenance. Most likely the building is paid for at this point and is completed owned by the Lodge group. That's why that can't sell the building. Their monthly debt service on the building is probably $500 or less and getting another meeting area for their group would cost them much more. There's probably other factors as to why they're not willing to sell also but I'm sure the low cost of ownership at this point is a good reason not to sell.
The reason they should let you get away with only paying for the utilities, insurance and some building maintenance cost is because you've increase the value of their building by at least the cost of your renovation if not triple the cost.
Concerning getting the money for renovations. Assuming that you keep good accurate accounting records of how much you make each year you should be able to get a loan for $50k pretty easily.
The net of this is that assuming you negotiate correctly and spend your money wisely I would estimate your cost to be about $1,800 per month for 5 years. ($21,600 per year) To cover the lease expense, other show related expense and staff (assuming you have some paid staff people) you probably need to be making some where between $60,000 and $150,000 per year.
To make $150,000 per year you would need to have 75 performances, with 100 people per performance and selling tickets at $20. The other way to make $150K is a combination of performing yourself and renting the theater. Because you have exclusive rights to the theater (and a newly renovated theater) and the high school is your only competitor you should really be in a great position to beat your competition hands down on renting your facility.
At the end of the day the game is to keep your monthly fixed expense as LOW as possible and your monthly cash income as HIGH as possible.
All my numbers are of course just estimates but I think they'll hold together pretty well.