Having operated an Aztec E for about eight years, I cannot praise it highly enough. It is enormously stable, solid and reliable, can be operated out of minimal runways (I accept 470m hard, 700m grass) is a fabulous load lifter - I have done a practice EFATO with 5 rugby players and full fuel and (admittedly on a cool day) it went up like a rocket.
The difference between an Aztec and a Seneca is that between wood and tin, or between a Merc and a Skoda. Nothing you can really measure, just the feeling of solidity and avuncular reliability.
It is true that Aztecs are very cheap. However, maintenance need not be too bad...Singh at Biggin knows them well and always seems to be able to source spares...and fuel burn is in the normal range, 100l/hr gives me 165kt TAS, or I can go slower for less.
The Aztec is a compromise between a Maule and a C421, but, to my mind, has the balance of the best of both worlds. Irv Lee wrote a good review about six or eight months ago in Flyer.
I would say that, in order of preference, the models are E, F, D, C. If you do get an F, look for long range tanks and the second hydraulic pump. If you do get the LRT option, remember that it limits your payload.