How much grunt do I need to run 2-3 simultaneous high bitrate 4k streams?

[TheDreadPirate]

* How much am I gonna be looking to pay up front? How much total?

Depends on where you live and what you end up choosing.  But what you're specing for does not require a lot.

* Can I delay some of that up front cost by just building a NAS first, and running JF from my PC? then buying a media server later?

A NAS is just a PC in a specialized chassis.  IMO, using a NAS just for storage is overused and adds complexity that a lot of beginners have trouble navigating, especially when it comes to file permissions.  You do not need one, especially if you are just using it for Jellyfin.

* Can I delay some of the costs. Say put in 4 hard drives initially, then put in another 4 later in later?

100%.  Gradually expanding storage is possible.

* Do I need transcoding?

Aim for direct play, but plan for transcoding.

* Should I get something rack mounted? Or is old cases n stuff fine? I half like the idea of having a mini rack, having like a NAS/Media Server/file server/Game Server/security system all running out of the garage or whatever.

You DO NOT need rack mounted anything.  Rack mounting just helps with density and organization if you have multiple PCs/servers.

* Should I just take a swing on an old second hand server/NAS or build/buy new?

Depends on your budget.  One thing you will need for your expected use case is a dedicated GPU.  Luckily Intel Arc GPUs are both cheap and extremely fast for transcoding at good quality.  You could buy a surplus office PC, making sure the chassis can accommodate a GPU (both power connectors and height), and just throw in an Intel Arc GPU.  Or if you happen to have parts lying around, as long as they aren't ancient and it has an available PCIe 16x slot you could use that.

What is your budget, BTW?

* If I build a raid array, with some level of redundancy, can I say for example have like 4 16gb iron wolf pros, with 1 of them as redundancy? Can I add in hard drives to that array later on to expand it?

If you use ZFS and configure as a RAIDZ1, you can just keeping add hard drives to the pool and it will accommodate them.

* What sort of minimum specs do I want? What are the recommended specs I want?

A not too old CPU.  At least 4GB of RAM if you are running a headless Linux server OS.  8GB of RAM if Windows or a desktop Linux OS.  The OS and Jellyfin's data directory should be on a SSD.  A SATA SSD is fine.  Just to clarify, this is only the Jellyfin app data.  Your movies/shows are ok to be on a HDD.

* any other questions I forgot about? lol.

Just need to know what your budget is.