The DVD drive should have a SATA connector already.
OP you can do this, I 3D printed a couple adapters to fit 3.5" drives into my old server case's 5.25" slots while migrating everything to a new server. My only real concern with the whole thing is that there's no rubber isolators on them which could cause issues longterm.
Harbor Freight has come quite a long way and has multiple tiers of product quality now, including stuff that can be compared to SnapOn, Matco, etc (the Icon line). Some of it is really good without requiring you to pay insanely inflated tool truck pricing and the rest of the market outside of tool truck brands is also built in China now so it's fairly slim pickings if you don't want to pay outrageous prices. If you use your tools for a living, there's a good argument to use mainstream brands like SnapOn for their convenience, but for hobbyists and the like HF will suit you well. For power tools I mostly use DeWalt mainly because they're better quality and because of the battery lock-in, but I do have a few Bauer power tools and they work great and cost half as much along with batteries being half the cost. DeWalt wants you to pay $100 for four 18650 batteries which is insane. For stuff like griding disks, cutoff wheels, or other consumables, I do tend to go with name brand since they're all universal and the cost difference isn't that much.
I do agree to be careful with stuff like jackstands, but I always double them up keep the jack under the car, put a tire under the car, and avoid being completely underneath them when possible. I actually bought a set of ramps just to avoid having to use them when I don't need the wheels off.