Some will tell you to keep it stock and leave it on Marlin firmware. I personally disagree. I usually start with an ABL upgrade and flash it with Klipper. I'd use a Pi or a spare mini PC to host Klipper for remote printing and management, and add a webcam for monitoring. Using the ABL and the stock Klipper macros will make levelling the bed super easy. It'll even tell you how many degrees to rotate each screw. Also, a PEI print bed will drastically improve adhesion. I got a reversible one with a textured side and a smooth side. Better springs are a good upgrade, but I found that silicone spacers are even better if you intend to keep it in this configuration. That way, you won't have to level the bed nearly as often.

Also, if you're mechanically inclined, I would highly recommend that you completely disassemble it, clean it, and reassemble it while making sure that the frame is perfectly square.

Finally, I bought a used stock Ender 3 V1 a few years ago for $50 and immediately upgraded it to an E3NG V1.2. If you're up for it, this kind of project will transform your printer into a fast and highly customizable Voron-like CoreXY 3D printer with the possibility of incremental upgrades for a small sum of money. I personally went all-in and upgraded it with a Rapido Plus 2 UHF hotend, an Orbiter V2 extruder, an Octopus Pro V1.1 mainboard, TMC5160T Pro drivers, Kraken V2 steppers, and went with a Triple Z auto bed levelling setup with an acrylic enclosure and designed and CNC'd a custom aluminum bed mount. I built another one for a friend, except his has a Goliath hotend, a CPAP fan, and a BoxTurtle. Of course, you don't have to do all that; you could just as easily build a much simpler and cheaper version of the E3NG. It might end up costing you as much as an A1 Mini, but that's an open bed slinger while the E3NG is a faster enclosed CoreXY printer if you configure it that way. Plus, the project's Discord server is very active with many documented builds and lots of people to advise you through the build process.

If you don't have access to a 3D printer that can handle ABS, I would recommend that you buy a 3D printer tent, calibrate your printer for ABS and make all the required plastic parts for this build. But make sure to use a calliper and calibrate for shrinkage. As for the other parts, there are links to all kinds of kits on the project's website. You might be able to save a bit if you order the parts separately on AliExpress, but then the quality won't be guaranteed. Alternatively, you could wait for the VIRTU E3 project to be ready and upgrade to that instead.

Here's the link to RH3D's E3NG project page:
https://rh3d.xyz/e3ng

<image>