The interesting thing is the Navajo's without wing lockers are lighter and either higher ZFW or MTOW (I cant remember exactly, it's been a while) but they will carry more.
Aftermarket Vortex Generator kits are fitted to most but not all Navajo / Chieftains in Australia. The fitment of a VG kit allows an increase to the maximum takeoff weight. The actual increase would vary between kit manufacturers but if my memory serves me correctly the MTOW on a PA31-310 Navajo goes up by 154kg (no wing lockers) or 100kg (with wing lockers).
The "Red Leg" kit referred to in a previous post involves some strengthening mods to the undercarriage. This mod allows an increase in the MLW up to that of the increased MTOW with the VG kit fitted.
I'm not sure what you are planning on using the aircraft for. You would be able to get your Aztec in and out of shorter strips than a Navajo. As far as the undercarriage being as robust as the Aztec, the PA31 is configured differently. Main wheels retract inboard as opposed to forward in the Aztec and there are hydraulically actuated gear "flipper" doors and nose gear doors in the PA31 which add complexity to the gear system. I would expect the PA31 undercarriage to be more maintenance intensive than the Aztec. At least you will have two hydraulic pumps!
There is also a PA31-325CR?? A Navajo with more powerful engines and counter rotating propellers. I have never flown one but think there are a couple around.