I actually agree with you - a well sorted Sprint was and still is a special thing. My one is absolutely revving its nadgers off at the moment, and flying along.
SDi is a common error, but SD1 is what Rover called its big 70s-80s car in-house. The car was not sold as the SD1, but as the Rover 3500, 2600, 2300, and 2000 (plus Vitesse and diesel variants). The proposed SD2 was to replace the Dolomite, but was stillborn (it was a mid-sized hatchback that looked a bit like an 80s Citroen or Astra). Some called the Honda Ballade-based Rover 200 series the SD3, but that was not official, AFAIK.
The Specialist Division was in effect Rover and Triumph, and the aim was to make upmarket and performancy cars.
The Rover 2300 and 2600 straight six SOHC engine is a Triumph engine branded as a Rover engine .
The 3500 is the Buick-based but Roverised all alloy light and compact Rover V8, also of Range Rover and TVR fame, tunable to ORL TEH POWAH. In twin plenum Vitesse garb, it is a Space Shuttle powerplant.
The 2000 is an O Series SOHC with twin SUs (they all have twin SUs until the fuel injected V8s ones at the end).
The 2400 turbo-diesel is Italian, IIRC.