Oh and to just add retropie has some quite nice crt shaders if you do go for lcd.
This is my short version.
is there an easy and not to expensive way of outputing to scart or say vga (an old pc monitor), or do you think the crt shaders could be a nice way of emulating those scanlines?I used the Gert VGA 666 adapter. Works pretty well with filters. I would use an LCD over CRT. PC CRT really doesn't have visible scan lines like an arcade monitor (low rez). With shaders it will not matter much. Viewing angle is better on the CRT but won't matter mounted horizontal. Vertically you can use an IPS LCD to fix the viewing angle or just mount the crt vertically.
im confused on how to go with connecting to the pi3, via an encoder usb or via an encoder trought the gpioThere are a ton of options out there. You loose the gpio on the pi using the vga adapter so you have to use a keyboard emulator. I have used a lot of ultimarc products. j-pac for hooking into a cab that is already jamma. i-pac for standard cab.
If the cab has speakers you will probably need an amp to power them or replace them with pc speakers. This hobby can get pretty expensive even just converting a cab. I-pac will do pretty much anything you will need it to do, is simple to hook up, has great support, software, and is commonly used in projects. Saves time and headaches so it's worth the money to me.
I just dont know what joystick and buttons should i choose.Bat top or ball top? Mounting to metal or wood?
Bat top you can just go with some happ competition joysticks with happ buttons. basic setup. mounts well to 3/4 wood.
Sanwa has some nice joysticks and buttons but mount to metal. So you might have to come up with something to mount them to something thicker.