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Q1. What graphics card should I get? I see the HD5450 recommended a lot. Looking to get all those low rez/Fq outputs via analog output but beefy enough to run frontend smoothly and maybe some newer fighters.
Q2. What Operating system?
Q3. What frontend? I see groovyMAME recommend wit CRT Emudriver but I do have a license for Launch box/Bigbox.
Q4. I do have a gaming pc (2700x/Vega64) that is right next to the cabinet. Could I run dual cards or would it be better to just use the old PC? I mostly run Iracing on my gaming PC and I am a bit worried about all the usb cabinet controls interfering with my wheels/pedals for Iracing.
I'm no expert and basically in the same boat as you (currently bringing and old long neglected MAME cab back to life), so take my opinions with a grain of salt.

Q1. No advice other than AFAIK GPU doesn't matter much to MAME, it's your CPU that counts
Q2. GroovyArcade (Linux) or Win10 with GroovyMAME/CRT Emudriver (basic GM info
here)
Q3. Launch box/Bigbox should work with GroovyMAME
Q4. I think using the old PC dedicated to your cab would be simpler