Quest 2 = $300 (64GB) / $400 (256GB)
I recommend the higher capacity if you are going to download a lot of games
Deluxe Audio Strap (DAS) = $100
(this is the best headstrap, the one it comes with is garbage)
DAS Adapter = $20
(the DAS was not made for the Quest so you need an adapter to get it to work)
Facial Interface or Silicone Cover = $14-$30
(both will block out more light, increasing immersion; the silicone cover is good to protect the headset from sweat if you are playing workout games...which I recommend trying)
Controller Grips = $30
(highly recommended as it increases immersion by letting you fully open you hands without dropping the controllers on the ground; make sure to read the full installation instructions or you WILL break something—the Quest controller, your TV, or your computer)
CASEMATIX Hard Shell Oculus Quest 2 Case = $25
Replacement Headstrap = $24
(the DAS is too bulky for my travel needs)
If you still have money to spend:
Controller Cover = $15
Helps protect controllers and walls from damage
If you have a gaming PC or are willing to buy (or build) one then consider the Valve Index for it's superior field of view and resolution. You won't have to buy any replacement headstraps as the the one it comes with is arguably the best available for any headset.
The Valve Index is $1000 though and does NOT have a wireless option, which the Quest is by default—however this can be mitigated with an ceiling-mounted cable pulley system which reduces the amount you notice the cable. The Index requires external tracking sensors to be mounted in opposite corners of the room. It's a more complex setup than the Quest. As soon as there is a wireless Index option I will be upgrading.
If you want a wireless PC VR headset consider the HTV Vive Pro with Wireless add-on but ensure you have a free PCI-E slot for the PC wireless receiver. I do not have a free slot so I couldn't use this option. Having tried both the Index and the Quest I am seriously considering rebuilding my PC to add another PCI-E slot and switching out the Index headset for the HTV Vive Pro headset and keeping the Index controllers which are better than the Vive wand controllers (Vive and Index system components are mostly interchangeable).