You don't have to change much.
For now, you can leave the original power intake and ditribution as is, and utilise it. I expect you're planning to run your PC & speakers from a power strip? You can just run the power strip to a normal socket to get yourself up and running fast, or....
You could hack (probably need to cut wires) the power strip cable end to a spare AC outlet from the internal power distribution board instead, assuming there actually is a spare spot. If there is no spare, you can disconnect the DC PSU and use that AC access point.
You should probably disconnect the DC PSU regardless if using a PC. DC PSU has nothing to do with the monitor, and most other things that require 5v/12v can be powered from a PC PSU.
The marquee is probably tapped into the AC power distribution already so may as well leave that there. You might eventually want to replace the tube/starter, or even replace the whole lot with an LED unit.
This is all assuming your original cab power intake/distribution etc is already setup well, up to the task (of taking a PC load) and not significantly degraded (e.g. by rats gnawing on cables or something).