ADS1256 from TI has eight single-ended 24bit channels with high-impedance input buffer and PGA. OpenEXG project has PIC code to interface it (they use two channel version ADS1255, but it should be the same).
If you want differential inputs, then there is ADS1298, with 8 channels, PGAs and A/Ds, internal reference, plus ECG/EEG circuitry which you can ignore. I am not sure you can find any example code for this one, though.
If you are looking for resolution, then precise, low noise reference is a must.
Card edge connectors can be ordered ruggedized, and with several platings of choice, check for instance Sullinscorp
There are ISA slots with a cap for protection.
Some vendors are quite willing to help you with selection for parts, if you describe your problem to them.
You may want to insert an empty daughterboard into the connectors when they are not in use, if you cannot use a cap.
Make your design so you can easily exchange the backplane too.
Usually, gold plated contacts last a very long time. When they didnt, we used a piece of cardboard with a very fine sandpaper (1200 or higher) laid over one edge, and inserted into the slot once. But only as a last resort, back when industrial PCs where still expensive. Sadly, I found no documentation about the connector lifetime.
Also, for extended outdoor use, you might consider a fully sealed enclosure. Filters tend to clog, nasty ants will move in, or even the dreaded micropenguin might build a nest inside the enclosure, if the climate is cold enough.
Delphi Connection Systems has some great products that are designed for automotive applications and some are waterproof. Mouser carries them. Eg. their Weather-pak series. They're used for pressure sensors and such like.
You can also use military-style circular connectors or M8, and some of the Lemo connectors are very good.