Since you're supplying to "all apartments" you'll probably need to set up a commercial account (for MDU) with Dish Network for legal purposes.
On the financial side, the equipment needed could get expensive depending on what kind of quality and features you'd like to have in the distribution. But on the technical side (and not relating much to cost), it could be kept pretty simple...
My suggestion: get as many 4-channel modulators as you need for however many channels you'd like to set up. You mention 50, so 13 modulators will do but make it 14 to keep bad luck away.

Also, even numbers make things a little nicer; but powers of 2, like 16 modulators, could make things even nicer.
ChannelPlus (aka OpenHouse) makes nice units, in both stereo audio output and mono audio output (but both kinds take stereo input). I have one of their "residential" quad-input mono modulators (model 5545), and one of their residential quad-input stereo modulators (model 5645) and both work very well. See my signature for more information.
With those 14 or 16 modulators, you don't need to use up every input, leave a few unused in the event one channel goes out on a unit at some in the future. Anyways, just tie the RF outputs of the modulators together using standard splitters connected in reverse. With that many modulators I'd have to do the math to see which is best, using all 2-way, all 4-way, some 8-way, etc. (accounting for the attenuation through the splitter tree). After they are all combined, you'll need an amplifier (launch amp) to re-boost the output levels back up so they can make it through the long runs and splits to each apartment.
Note, if you use residential grade equipment, you'll need to leave [at least] 1 unused channel between every channel you are outputting on. Also, depending on the quality of the entire cable structure, you'll most likely have to skip over other channels if your local broadcast stations are strong enough where you are located, to avoid ingress.
Alternatively, you can use commercial grade modulators, which usually come in 19" rackmount configurations, but those are considerably more expensive. With that higher cost, though, comes higher quality and more features. For instance, you don't have to leave 1-channel gaps between your output channels when using commercial modulators.
... I'm sure others on here can present alternate ideas that are better in one aspect or another (or just entirely better than the above plan), so keep in mind the above is just one guy's suggestion.
And welcome to the site, Treecutter.
