Hey Spencer Larson,
I would have a chat with your Esri contact as they will let you know if they can help out with this sort of work.
Depending on your level of database knowledge you may consider just using a relational data store which you will need to build when you do an enterprise build. When you publish hosted feature layers you can specify if people can edit the data which will allow people to edit the data using ArcGIS Pro or a variety of other products (Collecter, Survery123).
For your servers I would recommend you start out with one as you grow the capability in your agency and give it 32GB of RAM and 4 cores. You can always split the capabilities off on to separate machines at a later stage.
When we talk about high availability, active-active, active-passive we are talking about what happens if the server breaks. High availability generally means that you double up on everything so if one machine breaks the other one takes over. This is a more complex build and requires more administration so I would advise that you be aware of it and should you need to host mission critical data you look into it.
When we talk about balancing the load there are a few ways people interpret that. WE do multi site machines which is each machine has its own capability on it (seperate Portal, Server and Datastore onto their own machines). Multi machine generally aims to remove bottle necks and allows you to spec the machine as required for the capability. You can also balance load using two machines that are built identically and then you can share traffic between the two which helps to alleviate load.
A single machine installation will be the quickest and easiest to manage and will allow you to experience/test the capabilities that Enterprise has to offer. There are plenty of tools that you can use to monitor its usage and determine when you want to upgrade it to a more powerful machine, multi-site or load balanced. How long a single machine build will really depend on your agencies uptake of the Enterprise stack.
If you have any more questions please don't hesitate to ask!
Thanks,
Ben