The term you are looking for is “parametric”. You might be able to find a design you need through the many 3D printing file sites using that term. FreeCAD and Fusion360 are open used but another is OpenSCAD if you are more programming savvy than modeling savvy. There are plenty of recipes for threads in there to make your relatively simple design request here.
Tons of good and free options, you just gotta pick which one looks easiest for you.
Keep in mind that any hobby printer, especially those under ~$/€1k are subject to needing some level of tinkering to get reliable printing. If speed is unimportant, any of the options you provide will be a good fit. You will benefit from running some basic tuning for each spool of filament to get the best print quality. The speed can be dialed in to reduce failures but not take forever.
Build plate finish is more of a personal preference when it comes to PLA. Textured plate will provide slightly better grip to prevent the print from popping off mid-print, but it’s a visible and tactile texture to that surface. It would probably be best to have a both a texture and smooth plate on hand long term.
Enclosure mainly comes into play if you print high-shrink materials like ABS or ASA. If you don’t like to tinker, I wouldn’t try to enclose a prusa or ender and would save up for a design built for it. If you don’t mind tinkering a bit, you can always add the enclosure later.
Buying a used printer isn’t a terrible option, but I would get eyes on first to ensure it prints as-is, especially since you want it to be plug and play. People sell printers for a variety of reasons such as upgrading, lack of interest, lack of funding, etc.
The class keyword exists for a reason and it has a perfectly fine use case when you need to make use of creating new objects. I think it may be disliked because people come over from Java assuming you need to define everything as objects/classes when we have modules and other methods of doing the same thing with a little bit less clutter. I recommend reading up on the underlying functionality and how classes and objects work compared to modules.
It depends what you are painting! If you’re looking at smaller models/tabletop minis, you should take a look at setups used in the minipainting communities. Larger models might benefit from different setups. I know I didn’t want to start cheap and have to upgrade later, so I jumped straight for the Iwata HP CS. The dual action is super nice to have (not a must have though). I’d also recommend spending more on a better compressor before a brush (I got a simple 1gal compressor from California Air Tools).