I bought Mrs TMX a 98 Rover 414 Si with a failed HG i replaced it with the upgraded one and the car covered well over 50k with no problems!
i also changed the rad & thermostat as these can be why the gasket fails. was a very very reliable car the only issue was the imobiliser used to play up occasionally
i always thought the K Series lumps were All 16V units!
great engines they will take some stick unfortunatly the Rover 400 doesnt handle! i sold ours and got an Astra CDX that needs a new PAS pump!
K series available as 8v as well, used in metros/100s, some base wedge 200s, and some early bubble 200s. Think the 1.1 was 8v only.
I had a couple of 400 saloons, both diesels, both on her company car lease scheme, so both well ragged

, never really noticed an issue with the handling - maybe the extra weight at front helped? Had a couple of 416 petrols when ours were in for repair (

), but I was always chuffeured, as there was some confusion over the insurance for the loan cars, so can't comment on the handling of the petrols.