I'll second the pressure tester. Tough to set the gun up right without it.
For myself, I like having the 4c eyes, although I couldn't say I've ever known them to be an in-game help. I just like having the nice stuff.
Engines, there's the stock engine, which gives you the smoothest shot but not great efficiency. There's also the Supercharged engine, for better efficiency but sacrificing some smoothness. I've thought hard about moving to the SC engine, but I like how the stock engine shoots, and I can't carry more paint than I can shoot on a fill anyway.
My Darq came with a Violent trigger, and it really does feel nicer than my son's S-class stock trigger.
Boards, a Tadao is definitely nice to have. OLED if you want it, or just an LED works fine, too.
Barrel kit is something I've always thought any serious player should have. It does make a difference to match your barrel and paint, both in efficiency and accuracy. Which kit is, of course, personal preference. I like the Freak, but I also like the Empire Sniper barrel kit. Plenty of good sets out there beyond those as well.