Not OP you replied to, but have done similar before. Answering your questions in order (at least the ones not specific to that scenario) to share some thoughts, which are certainly not answers by any stretch:

- setting up a mobile network is quite different to a WiFi network in some ways, but perhaps a little more in line with a complex large-scale WiFi network. Mobile networks are architected around a "core network" which manages mobility and user traffic, and a "radio access network" which connects the base stations to your core network. The "core" is a fair bit more involved than a large-scale WiFi network setup, at least in my experience. You will need domain knowledge of 3GPP networks, and that's sometimes hard to get hold of, at least outside of traditional mobile operators and vendors. That can be a barrier.

- In terms of maintenance, a well-built RAN should "run itself" for the most-part. You'll want some monitoring on equipment. If you are using modern software-based base stations, they will run on Linux. Treat this like any large at-scale fleet deployment of Linux servers. Keep them patched and plan your maintenance windows for reboots etc. Your core network itself will realistically be a bunch of Linux servers to provide network functions. You'll have the usual periodic maintenance issues keeping up a bunch of complex services that you don't always fully understand, and the temptation to never update anything, as "it's working right now".

- The number of people you need for installing depends on skill sets. You'll need some experienced riggers to install macro sites on masts by climbing the masts. But you also need people who can pour concrete to get masts into the ground. And people who can install masts. You'll need someone who knows some DC electricals and can get everything powered up. You'll need some radio knowledge to plan out the network and check the antennas are pointed in the right directions and with the correct tilts. If you outsource this to a subcontractor, they can probably get the same people who do commercial networks to do it, but that will cost you dearly, and you'll never quite know what happened or how to fix it if something goes wrong!

- To operate the network I guess it's not a huge amount different to any other complex IT system. If you build out your core network well, you can serve a few thousand users without too much trouble, with minimal people. Once you start to scale beyond a few thousand users, you have to scale up the core and architect it a bit better. There's some routine monitoring and maintenance, and the usual things you need to do in any production environment, but in general it's not too horrendous. If you go down the "big vendor box" route like carriers do, they'll take big complex managed services to keep everything in check, but have far fewer really good network people on-staff. The biggest issue you'll have in terms of staffing up to manage it is getting the breadth of knowledge and skills needed to have access to the right range of skills as needed - legacy telecoms can get complex fast. If you are having an issue with IMS for 4G calling (VoLTE), you might only need 1 person, but that's in addition to the 1 3GPP core expert you have, the 1 IP networking expert you have, etc.

- You need to put a custom SIM into every device, and set up a PLMN identity for the network, which is just a 5 or 6 digit number that identifies the network to handsets. The SIM tells the phone what network it should try to join, and contains the crypto keys used to do authentication with the network. You can often get a PLMN allocated by your national telecoms regulator, or use one in the 999/xx range, which are set aside for private, uncoordinated use.