There are four options for gun - stock, L'etranger, Ambassador, and Diamondback. L'etranger provides a lot of flexibility with your movement and is generally the best option IMO, especially if there's no chance of them disguise checking (either they are not holding last and nobody has spawned recently for them, or you've watched their demos and know they don't do it). Revolver lets you butterknife + 2shot Medics, and if you run stock knife and no cosmetics, makes you hard to disguise check. Diamondback lets you get another frag after you kill something, it's an underrated option. Ambassador is good at mid-long range, which makes it suboptimal in 6s, because after you get a stab, you should probably be looking to die as fast as possible so you aren't stuck on Spy.
There are 4 options for knife - stock, Spycicle, Big Earner, Kunai. Big Earner is generally considered the best because chainstabs and you aren't taking much damage anyway. The main advantage of 125 health is tanking stickies while backcapping last, and also, the defender who goes Spy and checks your knife will think you're not on Spy if you run stock. Spycicle vs stock is preference, it has never made a difference for me in 6s so I use the stock for the disguise thing. Kunai + Diamondback or Revolver is a good alternative to have a better chance of getting a second pick, but your low initial health means it's harder to backcap, harder to gun down something if the stab fails, and you will occasionally die to random pipes.
Invis vs CnD is dependent upon map geometry and how slow the game is going. Disguise checking doesn't check cloak so don't worry about that. CnD is better for breaking stalemates because people will predict Spy time, which is the time it takes for you to walk from your spawn to behind their Medic. However, the Invisiwatch's greater starting cloak is much more handy in transition fights. It also has a quieter decloak if you let the meter run out fully, which lets you decloak much closer than CnD. Dead Ringer is only useful for fake dying when holding last to bait a bad push from the enemy team.
with that said though, the only time the enemy team will be able to do that is if they're holding their own last (or for some reason somebody walks back to spawn) in which case they can already change class or hit their resup bind and then are able to see you even when invisible because of the team outlines you get ~10 seconds after you spawn (which im pretty sure can be refreshed with the resup binds/swapping classes)
You can refresh that with resupply, but it only works while you are disguised as the enemy team and not invisible, so you can counter it with aggressive decloaking with invisiwatch or staying cloaked with CnD and forcing the guy to leave resupply by pressuring with the rest of your team.
tl;dr stock revolver + CnD to push last, CnD to break stalemates, stock watch in transitions