Drive-Ins are tough to value as the land they sit on is more valuable without the theater.
That's true where people treat their drive-ins as cast-offs, like they did 30 years ago. Pay attention to them, and their customers, and you can have a first-class operation that's well worth having on the property.
Syufy theatres just learned this in California. After sitting idle for 20 years, they reopened one of their drive-ins. They're quoted as saying they should have done that long ago.
As for what the operaton IS worth, you need to add asking prices for what land is included. I wouldn't get extended out into a deal that only leased the property.... for sure not if the lease was likely to run out before the contract on the business.
Otherwise, you have a lot more to worry about than what shape the projectors are in. Most any booth can be fixed up nicely without a huge investment. Unfortunately, many prospective owners focus on this area because they don't understand the mechanics well.
Your visible amenities are much more important... location, zoning, seats, screens, concession layouts & equipment,
restrooms, taxes, ambient lighting (in the case of the drive-in), past community support, potential competition, etc, etc, etc. If all of these things passed muster, I'd take a theater with an empty booth & have a show on the screen in a couple of days.