Our place had been run for 20 years with remote management and almost zero upkeep... just enough to keep the top layer scraped off. The projectors didn't go with the sale (not that they were all that great to begin with), the lamps were the worst in the business, 6 out of 10 speakers didn't work, and the amplifiers that powered them tended to blow... frequently.
Downstairs, the place was selling burgers off of one of those little home serving plates, even though they had a full grill setup in the kitchen (not wired though). Out of 4 sales stations, only two had cash registers... and that was more than enough.
The place needed paint, the fences were falling down, the screens hadn't been cleaned or painted in who knows how long, etc, etc, etc!
We compared our problem to what a person might face if he went into the ferry business on a budget, and found a boat that needed paint and a new engine to get going. The budget would only allow for one fix at a time... which to do first?
In our case, we decided to replace the engine, then paint the boat. Painting first would have made it look good, but it couldn't go anywhere, so no income.
It's been an ongoing project (what indie theatre isn't?), but we redid the screens, upgraded the booth (several times), rewired the kitchen, added registers (eventually, a full POS system). The extra years have been tough on the old fences, but we're working them over now... replacing a few sections each year (it's a big place!). The increased business over the years has allowed us to add new boxoffice facilities, repaint, reroof in places and add commercial hoods, fryers and a walk-in cooler/freezer.
So... my vote would be to add the screen and make the front look as good as you can, then redo it later. You're doubling the offerings on your menu. Make the presentation nothing to complain about and I think most will forgive the age of the surroundings. It's the show that the customers are coming for.
... just my $.0002
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