There are alot of mods, you'll have to be more specific. There's a metallic construction mod, there's ACCM mod, TWM mod, plain construction mod, there are many
Dark Dragon DX
You could get a huge pack of Tribes 2 stuff here. Some of the mods in this pack you'd be interested in are as follows:
Although the mod pack is pretty rediculous if you're just looking for a combat-con type thing. If you wanted to get just any of the listed mods above (there may be others, I dunno, google around), here's individual links:
There's also Moocon, ECM, Metallic and some other non-combat oriented construction mods if you're interested in them as well. They are available here. (Check the archive/ directory as well).
Now I was just assuming you wanted a combat-con type game, if you wanted some other thing then you should just look around in the Tribes 2 Ultimate Pack I linked above, there's plenty of mods in there. Of course, you could also just not run a mod at all (base) or run Classic as well.
As far as running the game offline, as long as you're behind a NAT you could simply not forward any ports and run the server in an online session as it will not show up in the server listing and can not be connected to from outside of your network.
If you really wanted to force offline mode, you'd just need to run Tribes2.exe with the "-nologin" parameter -- not the "-online" one like you'd normally do.