Tamiya never made a 512. And "these old school kits" easily put to shame the modern Revell or AMT kits.
I am pretty sure Robustelli are long gone. They were never really all that great in the first place anyway. Where they were irreplaceable was the 1:18 scale diecast. They made kits that would transform a Bburago toy into a decent model. Simply because back then (late '80s, early '90s) there were no high end diecast models.
Great kit you have started there. Few pointers if I may.
I would join Alex on the color. A Testarossa has to be red. Altho I would agree with you, it's one of those cars that look great in any color. Except in yellow. I grew up with yellow Testarossa near my house. As much as I loved the car, it just did not look right in yellow.
About your engine parts. Considering how these cars were used and maintained to this day, you will not find one with engine parts in the colors you used. The valve covers and the red parts on top of the plenums need to be flat red, not glossy. And then the block and the plenums themselves should be just a plain flat aluminum. A very light dull looking almost white color. If you have, or can get, a spray can of Tamiya AS12 - Bare Metal Silver, it would be the perfect color for it. And I would not use any wash or anything. These engines never got oily or dirty or anything like that.
About photoetched parts, there is plenty to be had on e-bay. Just search the seller "globaltoy" and search "ferrari" in his store. It is in China, but buy with confidence. Good seller I never had issues with. A couple of sheets of his emblems would cover your Ferrari builds for quite some time. I personally would stay away from photo etched wipers. They are total pain to fold to create a 3d shape and then the plastic kit part from Tamiya, still looks more realistic. I would just hand paint the mufflers with Detail Master Stainless Steel buffing metalizer and rub it out.
Have fun building this kit, which with not much effort can be greatly detailed.