The workhorse undercoat: matt black primer in the 400ml army-size can. Black-priming is the classic route for anything with built-in shadow - locomotive chassis, undercarriage, track sections, dark rolling stock, and any model where the recesses should stay dark.
Prime here, then one-coat with the Speedpaint Starter Set; seal it all with Anti-Shine Matt Varnish.
Do I still need a separate primer?
No - that is the point: Colour Primer is undercoat and base color in one pass, saving a whole painting step.
Speedpaint over this?
Perfectly - a matt Colour Primer base is exactly what Speedpaints want.
Shipping note
Aerosol product - ships ground service only (no air or international shipping).