The MK3 does have auto Z leveling, but it does it with a PINDA sensor, and you're still going to need to do a Z calibration plus maybe some fine tuning, and that's not the hardest thing in the world, but it's not the easiest.

The loadcell on the MK4 completely eliminates that whole mess.

I've put 10's of thousands of hours on MK3s and the PINDA works fine as long as it is mounted well (which isn't always the case) and set to the right height relative to the nozzle. The automatic calibration routine - on a well built printer, which also isn't always the case, runs exactly once after you've built the printer. Fine tuning is possible, and can be quite useful if you want to play around with the degree to which your prints stick to the bed, for instance for automatic print release tricks (and bed temp as well).

The fact that you have all this control comes at a price compared to say a Bambu A1: you need to know a bit more about how a printer works. But that knowledge will come in very handy when sooner or later you want to do something that isn't in the manual.

The MK4 loadcell is a solution for some problems and a the cause of some others, it is not the best implementation of a loadcell either but it is smarter than the way for instance the Creality K1 and variations are set up (they have four, underneath the bed).

I would recommend against the Creality series printers by the way, they're dirt cheap and wicked fast but they break all the time and the stock extruder is complete junk.

Finally, the best investment you can probably make alongside any printer is a multi-roll filament dryer with a heating element and a fan. That more than anything else will affect your print quality.

We do the bulk of our work on Bambu's now, but for really high precision stuff we will use the MK3s and it's amazing what those old printers can churn out. Nozzle wear is a bit higher on the MK3s than on the Bambu's. The oldest MK3 we have has done > 100 Km of filament...