They aren't slow, but they certainly aren't as quick as a 20VT in a straight line. The handling however is excellent, much better than the coupe IMO.
Timing belt changes are a real PITA and consequently expensive if you use a dealer, but that shouldn't be such a worry if you are coming from a coupe! Slightly more worrying is that conrod failure is not unheard of in high mileage engines, though it's not a common problem. The KV6 doesn't tend to suffer from head gasket problems like like the 4 cylinder variant does.
Personally I think they are good value cars at the moment, I wouldn't hesitate to own one.
As JimO said, I have also heard that the V6 ZS is very well sorted. Aren't they RWD?? Look a bit boring (IMO) but apparently a hoot to drive and very involving, bit of a sleeper.
The only RWD Rover/MG is the 75 V8/ZT 260 and I'd swap my coupe for at ZT 260 in a heartbeat, though might need to re-mortgage the house to pay for fuel.
This is a video clip of a friends ZT 260 with a Dreadnaught garages supercharger conversion which ups the power to around 400bhp. The rather battered looking Pug is also an interesting car, just shy of 270bhp from a normally aspirated 2.0 liter engine, and it only weighs about 650kg.