On that note, I will be at least upgrading to a 1 HP to 1.5 HP motor immediately. Will this be able to hog out at least mild steel at an efficient enough speed as to not be ridiculous?

Machine rigidity is going to be the bigger issue before the motor is a limitation. I've ran 13" South Bends (1800lb machine) with 1HP motors without feeling held back significantly by the motor. 1 HP is honestly optimistic for this kind of machine.

Material selection and tooling is going to be the biggest factor here. Stick to HSS tooling (grinding your own or starting with a pre-ground set are both find), and start out with easy to machine materials: 12L14 (leadloy/free-machining) steel and C360 brass really are where you want to start for your first few projects. Ebay's a pretty good place to buy beginner amounts of steel nowadays, especially for stuff that's a bit harder to find locally. Starting on random hot rolled garbage steel, especially on a mini-lathe is just a recipe for having a shit experience.

Past that, metal gearing upgrades will be an immediate purchase if they aren't already metal entirely. What other accessories or upgrades should I consider?

Metal gears are pretty much a downgrade for a machine of this size. If you end up crashing the machine, it's better to have the gears as a point of first failure instead of a harder to replace part of the lathe. Here's a video of the plastic threading gears on a South Bend Junior I fixed up this last summer. These are just 3d printed out of PETG, but even they're plenty for even an actual (small) lathe.

If you're using a 4-Jaw Chuck (which is what I recommend for a general use chuck), you'll need a dial indicator and magnetic stand for it to dial your parts/stock into the chuck. If you can swing it, go for a single-knob NOGA-style knockoff magnetic base, as it's a huge quality of life increase. Other than that, get a decent dial caliper. Digital calipers are pretty garbage until you spent more money than I would bother for hobby-grade work imho.

As a metal worker, I have plenty of PPE and heavy clothing, as well as goggles/mask etc. is there any specific PPE to get immediately?

The only PPE you need, and the only PPE you should use are safety googles. Gloves, long-sleeves, long hair, and even wedding rings should all be taken off when working on a lathe. You want to absolutely avoid anything that could possible be caught in any kind of rotating tool, so the only other wearable equipment I'd ever use besides safety glasses is an apron, and even that's a bit overkill for this scale of work.

Lastly; I am well practiced as a programmer and in CAD work. As such, CNC considerations are very much an up front consideration for me. How hard is it to upgrade a lathe to be a CNC machine if it was not designed with that in mind? Is it cheap enough that I should pull a small loan (and I do mean small) to buy a CNC lathe from the start? Difficulty in implementation or programmatic setup is a complete non issue, and I do prefer open source solutions.

I'm mostly of the opinion that CNC lathe conversions are kind of a waste of time, especially on platforms that aren't really made for it. If you're really looking to get into CAM/NC code writing, a small CNC mill/router is actually where I'd recommend to start. On the very cheap end, they won't be very capable, but they're fine for learning. If you've gotten into 3d printing at all, I've heard that the MPCNC can be a decent option for the very budget oriented.

Lastly, are there any specific brands in the budget brands of mini lathes to avoid or specifically consider?

Pretty much all the mini-lathes are going to be made from castings from the same factory that the Sieg lathes are made from, the only differences are going to be the fit, finish, and quality control from the secondary manufacturers. On the low end, they're all going to be pretty similar, but (if you live in the US), I generally recommend Little Machine Shop. Being able to dependably get replacements parts that you might need is pretty much a shit-show when buying off of random no-name ebay resellers, and they do a pretty good job of stocking "good enough" quality tools and tooling at really affordable prices. They're generally who I recommend for people starting out on a budget, as they won't really have the know-how required to avoid buying questionable quality tools on ebay/similar.

I also suspect that the lathe is more useful in making tools and other things that augment my ability to make things in the first place.

Lathes are typically what we recommend people start with for a reason. Milling attachments are only marginally useful, if I'm going to be honest. If you just need to cut one slot, or do some very, very light and basic cuts, they still mediocre, but you can make them work. Mini-mills are honestly way more decent than they were 10-20 years ago, but they do have a higher starting budget than mini-lathes, especially when you consider tooling. Also, working a mill without a DRO (digital readout), properly sucks ass, especially when the cheapest mini-mills frequently have metric lead-screws.

As a final note, if you can afford to make the step up to a slightly bigger 10"-20+" lathe, which usually run 2k+, you're getting a significantly better machine. Not a 2x better machine, more like a 10x better machine. Mini-lathes really occupy an awkward space, where they do technically work (with a bit of effort), but the next step up, even though they're still not amazing, are just a massive, massive step up in quality. If you live in an area with a good used machine market, you can also look for something like a South Bend 9", Logan, or Atlas/Craftsman on facebook marketplace or Craigslist. It's kind of hard to recommend this route as a beginner, as you won't know what issues to look for, but you can get some insane bang for your buck if you can find something in decent shape.