Definitely stay away from Bachmann, sort of. Stay away from the Big Hauler line. That is pressed metal rail. It will rust very quickly, it is not meant for outdoor use. Bachmann has introduced a line of solid metal track and that should be suitable for outdoor use. Its pretty new so there isnt a lot people using it yet.
The material for your track is really dependent on what you are using for an operating system. track power (DC power pack), DCC or battery/RC
If you are using track power or DCC, you will need a constant reliable connection between the track and your locomotives, which requires cleaning the rail. So you will favor materials that dont tarnish as much. Keep in mind all metals will tarnish in an outdoor environment. Some just arent as much. RC/Batt does not rely on a connection with the rails and so it doesnt require clean track.
Brass will tarnish pretty easily and so it is not an ideal material for track power or DCC. It is ok for RC/Batt (This the material I use because I use RC/Batt).
Aluminum is pretty good although it is not as durable as other materials. However, since you are elevating your rail off the ground this should not be an issue. I do believe that aluminum will oxidize pretty easily so you will have to clean it frequently for DCC or Track Power.
Nickle Silver is better than Brass in oxidation but you will have to keep it clean.
Stainless Steel is probably the preferred material for track power or DCC. It does not oxidize as much and maintenance is a lot less. It probably the most durable of all the materials when it comes to abuse. It is also the most expensive.
If cost is a concern, I would suggest looking at Nickle Silver as an option, if you are looking at running Track Power or DCC.
As far as materials for your ladder. My only advice is look at materials that wont rot away. I like wood and it is easy to work with, but you will have to replace it as it rots. It is cheaper so you have to weigh that out. Look into stuff like PVC which is more expensive. I used PVC 1 x 2s and PVC 2 x 2s with PVC pipe as my risers and it will last a very long time.
Thats about all the advice I can give you. Keep asking questions and we will help as we can.