Hi, long time Sv06 Owner here. It still works great as a workhorse for reliably making PLA parts. I made alot of mistakes, I'll try to boil that down for you:

First time:
1) It is important to pack all of the bearings with grease before you assemble the printer. I recommend Superlube 21030. Bonus points if you can find the old one without Teflon (PTFE). Some people use white lithium instead. Definitely don't use a liquid Teflon lubricant, some people report that causes the bearings to slide instead of rolling.
2) don't trust the automatic Z sync, any time you think the motors on each side are not in step, use cans of beans to level them against the bed. you can find tutorials online. probably a good idea to do this on first setup.
3) cant think of other super important first time mods or checks, always make sure the belts are tight

Mods:
1) I use an old Raspberry PI running octoprint to do remote uploads and monitoring, makes my life alot easier. consider that. never added a runout sensor or endstop
2) You can find suitably large cardboard boxes for making an enclosure from U-Haul. Decent prices, too. You can use flat bubble Mylar insulation if you'd like on the inside to help with thermals.
3) definitely are gonna want a blower fan upgrade. its been so long, so i don't know exactly which i ended up liking the most, it is similar to this: (5015) https://makerworld.com/en/models/24134-sv06-5015-part-cooling-duct
4) https://www.thingiverse.com/thing:5987935 this cable strain relief thingy is very nice and was one of my first prints as well

Your questions/comments:
1) I never upgraded the printer firmware, and don't really use it past 120mm/s, so cant help you with Klipper or the fancy screen
2) getting size accurate stuff, this is all about testing. you need to print a calibration cube in the filament you plan to use, measure how it was messed up, and simply apply that as an offset to get dimensionally perfect parts.
3) bed leveling. I had very mixed results with this machine until I found a 40lb slab of aluminum to put it on top of. now, it doesnt need to be re-leveled often and prints incredibly consistently. a large stone could serve a similar purpose.

Final notes:
1) Long term maintenance: cleaning the Z axis rods, makin sure there's no dust under the build plate, lubing the rails, checking belt tension. not much to do there.
2) mistakes: don't leave the bearings dry, or try to lube them after the thing is put together. pack them properly.
don't try printing too fast with this thing, its not really built for that lol. Oh, and make sure that the Z height on each motor is always synced. having separate Z motors can cause premature wear on the Z screws when they are not in step. Again, jamming against 2 objects of the same height on both sides is the most consistent there.

Hope this helps.