Good cars IMO, based heavily on the Rover 75 but with upgraded suspension and other bits.
The K series engine was in the ZT, in 1.8 turbo and NASP form. It is fundamentally a good design, however obviously plagued by headgasket issues. Once rectified properly a good K engine should do 50k+ without another gasket, and this issue aside is extremely reliable.
Personally I'd be looking at a V6 engined model, using the very reliable (and not HGF suffering) KV6 engine.
The BMW CDTi engine is good, but MAF issues have been reported, aswell as these being fitted with an expensive Dual Mass flywheel assembly.