No need for primer, I also painted my astro Testor red ,cab & frame.
First apply a "dust" coat; paint pigments will give a good grip to futur paint coat, then apply several light coats until you reach the finish you want, be sure to have a a good holder when spraying your cab, remember light coats will give a much better finish with enamel paints.
The Astro 95 kit was introduced back in 1971, with as mention earlier a turbine engine option included with the actual engine, like many truck modelers the turbine engine was long awaited but never came back ...only on eBay at ASTROnomical prices!
Since the kit molds where modified by AMT in the mid 70's to update the astro: new mirrors, cab grab bars & step laders, top airfoiler, seats, etc, it's possible that the turbine engine was "removed" from the retooled parts sprues.