I believe you're looking at this the correct way. A well-maintained 420 can be properly outfitted for far less money
than a new 42. One other thing to think about perhaps is resale. The 420 and to a lesser extent the 421 have not held their respective values as well as other Lagoons.
The Lagoon 420
series (420, 421) are both very good liveaboard
cruising catamarans. The early issues with the hybrid engines and negative opinions about the 420's sailing abilities affected, and continues to affect, the way the 420 is viewed by many sailors. We looked seriously at a couple of 4 - 5 year old 420's before deciding to purchase
a new Lagoon
42. Potential resale value was important to us. The Lagoon 420
and 421 are essentially the same boat
and although a few improvements were made in the 421 it is little more than a re-branded 420. The 420 was re-branded the 421 because of the issues that hounded the 420 primarily because of the failure of the hybrid engines to function properly.
42 is a completely redesigned boat and is, in fact, quite different than either of her predecessors. Thus far it is a far more popular boat than the 420 and 421 combined. Hull
#'s of Lagoon 42's being ordered now are approaching or in the 600's. The Lagoon 42 should hold its value (fingers crossed) much better than the 420 or 421 have.