Duplicolour Trim paint and Vinyl Dye = Your best/cheapest option. The trim paint has a matte/satin finish matches the dashpad/door card material closely. And the vinyl dye works really well on the headliner and sunvisors. Doesn't chip or crack. I had my bugeye's interior completely blacked out.
That held up for 3+ years of daily driving too.
For the seatbelts rings, I ended up wrapping the seatbelt in duct tape, and masking tape, fed it into the eyelet and sprayed them down.
Best thing to do is use duplicolour wax and grease remover and wash down all the panels completely, dry, spray.