I have flown the A, B, and C models of the 421
My take
421 A(1968-69): Stay away as it does not have enough wing so the performance is basically the same as a early 414 with way more fuel flow. It also has a low usefull load and narrow C of G range.
421 B(1970-1975): A very impressive airplane. King Air 90 Performance, good load carrying capacity and a very stable instrument platform. Downside is the needlessly complicated fuel system and a unreliable pressurization system on the early ones (1970 to 1974).
421C (1976-1985): The best of the breed. It has the late model high diff pressurization, simple wet wing fuel system and the best performance thanks to its clean wing.
Overall I think the 421 is the best value going. The 414 A (basically a 421C with direct drive 310 HP engines) has less performance and a poor usefull load but cost more year for year. This is mainly due to the IMO undeserved poor reputation of the 421's geared GTSIO 520 engines. The 421C I last flew was sold with 1450 hours on both engines and all original cylinders , turbo's, exhaust (except for one precautionary tail pipe replacement) and fine wire spark plugs. Except for 25 hr oil changes and regular fuel pressure calibrations (vital for proper engine operation) the engines required virtually no work.
But and this is a big but , if a 421 was built today it would cost at least 1.5 M US $ . Parts are priced accordingly so you must have the money to run it. For example the owner of the above 421C I flew budgeted $ 40,000 a year for maintainance (not including engine/prop reserve) and it was in good shape when he bought it. He also budgeted 3500 US $ a year for annual simulator refresher training at SIMCOM in Arizona for both of us, another must IMO if you want to safely operate this class of aircraft.
If you are serious about buying a 421 buy the very best one you can find as it will be the best deal. Pay now or pay (more) later but you will pay.
Finally get someone who has a proven record of sucess operating the airpalne to teach you how to fly it and more importantly how to manage the engines. Proper use of the mixture control in all phases of flight, setting the correct RPM, and carefull CHT management, are necessary for trouble free engines.