Does this valve need any plumbing or can I bolt it straight on?

Installation method and mounting specifics are not detailed in the product data. Test-fit the valve against your turbo setup before committing to any fasteners or adapters — you may need custom piping or flanges depending on your intake design.

Will the horn go off every time I lift off the gas?

Yes. The integrated horn sounds whenever atmospheric pressure is vented, which happens any time you release throttle in boost. If you want silent operation, this isn't the valve for you.

Can I paint this to match my engine bay?

The valve ships in black and red finish. If you want a different color, light sanding and automotive spray paint will work, but test on a hidden area first — the finish durability under heat cycles from exhaust proximity isn't specified.