I'd build the MK3. It's a workhorse of a machine if you treat it right. After a while if you decide that first layer calibration is grinding you down or you need it to be a bit faster, you can buy an MK4 upgrade kit or build a Voron... or both

Build the mk3, take your time, because the better the build is, the less issues at end.

Then see if the hobby is for you or not.

I missed the MK3 part on my first read-through, but yes - the MK3 is still bangarang as-is.

I'm still sporting my original Prusa i3 from 2014, before they had any MK-notations. Must be the first batch actually sold through Prusa Research and not just through his workshops. I've dropped the 2.75mm filament and upgraded it to a MK2-style extruder. I've also made a rack-and-pinion setup to fold up/down a microtrigger on the extruder itself which I use for mechanical Z-probing.

Nothing wrong with a MK3 at all. The changes made since then are more nice to haves rather than big leaps forward. The printer will give you reliable high quality prints even if it is a bit slower than MK4 and has a tiny bit more setup for getting the nozzle distance correct.

I had an order in for the upgrade kit for my MK3S and quickly came to my senses that the benefits are minimal.

After looking at the upgrade options (and shopping with your wallet, not mine), I'd say buy a MK4 kit. You're in a fairly unique position that you aren't taking apart a working printer; you're getting a new printer with 2023 electronics and motors for a discount.

However, if sinking that kind of cash isn't wise for you right now, then build what you have! Or, pick up the MK3S+ upgrade kit for $50USD. MK3S+ is a reliable workhorse, and mine won't be retired anytime soon. It still prints PETG as well or better than my X1C, if a little slower.

Good luck and have fun with it!

I have a mk3s+ and it's so good, definitely more capable than I need

Just build it and enjoy. I would eat gummy bears anyway 😜

For upgrade kit its still better (cheaper) to have working printer since you can then print some parts and buy kit for $50 afaik. Upgrade is not big IMO

I have both MK3's and MK3S's at my college. About 4 of each. Functionally there is zero difference. The MK3S has a filament sensor so you can run spools to empty and safely replace without having to hover over top of it, but that's about it. They both print fantastically and with remarkably high accuracy- that is, when some college bonehead hasn't broken a hotend or completely ignored all calibration.

Build it and use it. She'll serve you well for many years to come.