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.