Welcome aboard, Frietbakker.

The wiki is down right now, but there is a fairly recent copy of the BYOAC FAQ on the
Internet Wayback Machine.
The control panel will be an RVS laser cut plate (2mm thick)
Not familiar with "RVS", but given the thickness I assume it is a type of metal.

i found a six button scheme online somewhere but it doesn't mention hole diameters.
Probably
Slagcoin.
- What are the mainstream pushbuttons and joysticks? There are so many products.. i'm not really familiar with original arcade machines (There is Sawna, ILSA, Seimitsu.. What is recommended?)
Depends on what types of games you like to play.

90's fighting games? SHMUPS?
- What diameter holes should i drill in my control panel to fit the controls?
Buttons are almost always 1-1/8" (1.125")
or 30mm.
Joystick hole size depends on the stick and how far from the top of the panel it is mounted.
A 1-1/8" hole is good for most sticks in a 3/4" wood panel -- you could use a smaller hole for a metal panel.
- How are these controls mounted onto such a plate?
Buttons either have a nut on the underside or locking tabs.

Joysticks on a metal panel:
- Carriage bolts are the easy way so long as you don't mind having the heads visible.

- If you don't want to see the carriage bolt heads, weld bolts/flat-head screws either to the underside of the panel or to a separate plate like Luigi did
here.

- Should i mount a wooden support under my 2mm RVS control panel plate?
Assuming that it is metal, bending a small tab (1/2"?) on each edge should minimize the amount of flex.

Scott