If you can trust to that used mk3, 10 days is nothing, I would go for that one. My average print time is 6h so it means someone printed like 40 prints and I could do that in bit more than a month of average use. They are made for way more hours.

High quality, no hassle. I want to print, not tinker.

Sadly, there is some thinkering involved even with $2k+ machines. Getting prusa over ender is like paying more to dodge desing flaws, faulty parts and crappy support, its not buying plug&play machine. Its still hobby grade machine. Most important is to have loads of patience and prepare for learning advanture, but thats super fun IMO. Once you master your machine, after you experienced many failed prints on it, you can then proceed to plug&play workflow.

IMO, assembling your first printer is highly recommended, it will help a lot in the future. Auto bed leveling is pain that you need only if hardware is not built in low enough tolerances, so if you can get printer that prints good without abl its even better. Speed is super important, print time is quite huge on even fast printer. You can expect 12h print for some 15x15x15 cm part with 0.4 nozzle and 0.2 layer height. And thats quite optimistic guess, some prints take days to finish. You will always balance between speed and quality. Using 0.8 nozzle can push 4x more flow with proper hotend if quality is acceptable. So its not just about axis movement speed or acceleration, its quite important how you model your parts and how good you are with your slicer. Using different infill or model orientation can be a big thing sometimes.

About size, smaller machines are more rigid but can print smaller parts obviously. Since printing is super slow, smaller is better for cheap machines imo, but kinda limiting if you want to print long flat parts for example. Prusa mini have cantilever X axis, so it has to be much smaller or use much higher cross sections for same rigidity as mk3. Rigidity is actually a limit for your max accelerations/speed without losing on quality.

Also, I wouldnt put filament sensor and power features on the list. Lot of people disable them due to issues they cause, it sounds more usefull than it is. 1 kg of filament is like 24 hours of print time and its not hard to guess if you have enough material for a job (slicer will tell you how much you need). If you lose power printer will stop and probably make scarf cuz heated nozzle will heat everything around it. If thats not on the visible part, you can messure your print and continue from there, or just print top part and glue them together. Its quite hard to continue without noticable mark afaik, and UPS is probably the right way to deal with it. I lost power 4 times in 7 years, found that a lot, but was lucky cuz printer was on idle every time.

Start with PLA since its the easiest to print and suits most people. PEI sheet with no enclosure will work easy, but make sure A/C or window is not blowing air on one side. Enclosure is still amazing, it protects printer from dust, isolates some noise and it makes environment temp more stable which is super important, especially for materials like ABS. It can also help in keeping your filament dry, especially if you want to print PETG (filament drier is recommended and almost must have for PETG anyway). Its safe to start without one, but deffo plan to get enclosure in the future (even cheap ikea lack will do).