I have about $5000 to $6000 to spend. As I said, it's not fixed.
That's what I'd have to spend on a decent portable DVD player with sufficient battery life and DVDs. I figured about 40 players and 50 movies, although I might need more players.
You MUST be missing a 0 in there if you expected that to cover 40-60 iPads + movies. Let's look at the costs here. The very cheapest iPad is the 16 GB mini @ $299 each. If we go with just the minimum number of iPads you need, that's $11,960 alone, + taxes. Now let's get to the movies. Let's say you rent them. You'd have to pre-download them because streaming while flying probably isn't going to work out. You have 30 days to view them from the time you rent them, and 24 hours to finish watching from the time you start watching them. Now, to rent Kingsman, for example, is $4.99 in standard definition resolution. You'd have to rent it 40 times on 40 different iTunes accounts. That's $1,996 to rent 10 movies for 40 iPads. And don't forget the logistics of activating the iPads; pre-downloading 10 movie rentals on each of 40 iPads (that's 400 downloads in this scenario); and managing the inventory of all these iPads on top of this. Oh... and we're assuming the 16 GB model of the iPad will actually hold at least 10 full length movies. It won't. The SD version of Kingsman is 2.31 GB, which means you can expect to need about of 23-30 GB of free storage space. So you'd need the next cheapest iPad, which is the $349 iPad mini with 32 GB storage, which will be a tight fit considering iOS does take up some of that space. So revise that iPad cost to $13,960 (assuming the smaller screen of the iPad mini is even acceptable, vs the iPad Air), + the movie rentals @ $1,996, + labor to manage all this, which I would guess would take a minimum of 16 hours of work for a single person to activate; configure; and download the movies to these iPads. This is assuming that this person has access to an internet connection capable of downloading 400 movie rentals in 16 hours!
If you go the portable DVD route, you are looking at $50-$120 per device. At the low end, that's 2 grand off the bat. Then you have to buy the DVDs. Let's just say you want to have a selection diverse enough to satisfy everyone, and have enough copies of each movie. It wouldn't do well to have, say, a single copy of a recent release if 5 of your clients want to watch it. So let's just say you need 5 copies apiece of 20 new releases. That's 100 movies x $15 or so, so you are looking at $1500. So overall cost here is about $3500, and minimal effort to manage this. You have more leeway with your budget, so you can spend more for nicer DVD players, or have a larger selection of movies available. Or both.