Frank Costin was an engineer who started his career in aviation design and later moved into automobiles and auto racing.

[2][3] In Costin's personal history of automotive designs, the Amigo was Auto Project XVIII.

[6] The goals set by Costin for the Amigo included the capability to cruise at a steady 100 mph (161 km/h) with an engine speed below 5000 rpm, the ability to cover 250 mi (400 km) without tiring the driver or stopping for fuel while carrying adequate luggage for the trip, and a rate of fuel consumption of 30 mpg‑imp (9.4 L/100 km; 25.0 mpg‑US).

It subsequently moved to Little Staughton, Bedfordshire, and finally to a location near Luton, where Vauxhall had a large factory.

[11][1] Parana pine replaces the Sitka spruce used by Costin on the earlier Marcos structure for jointing strips and local reinforcements.

[8] Rollover protection is provided by a triangulated steel tube attached to the double-boxed rear bulkhead.

[8] Its shape includes a reverse or reflex camber line like the one Costin had used in his aerodynamic refinements of the body of the original Lotus Elite.

[18][10] An unusual feature on some cars is a fin-like pylon that is attached just ahead of the trailing edge of the roof and is topped by a small red lamp.

[2][19][5] Costin's focus on aerodynamic efficiency meant that even items like the external mirrors were subject to rigorous scrutiny.

[2] The rear suspension employs a Vauxhall Victor live axle with leading arms, coil springs, and a Panhard rod.

[3][22] At this point the car had a 2.3 L slant four tuned by Bill Blydenstein to Dealer Team Vauxhall (DTV) specifications.

[3][23] The car was later completely rebuilt by Blydenstein, with a freshened dry sump 2.3 litre engine, a 5-speed ZF transmission and dual circuit brakes.