Well the only mods you can really get is the Halo Reach modding program called Liberty, if you google it you will find a link to it, it only works for campaign but is very easy to do. It can give you weapons and insane amounts of ammo, also it can make you invincible.
liberty is one of the best modding programs for halo reach and you can also mod halo aniversery edition too..google or bing it(liberty halo editor)
Yes, use liberty. Here's the link http://liberty.codeplex.com/ Just be advised that you will need to re-edit the save every time you start a new level. Also if you add a weapon in your inventory that is not available at that point in the game, it will freeze your game. I would recommend just doing ammo and health.
liberty is a good mod tool but you have to be careful when you use it. you dont wanna get banned, unless your using a dummy/offline account
there are other modding tools for usb, I mean some people have found a way to turn a mod for a jtagged or rgh'd xbox 360, such as gamesaves, maps, multi-player options such as slayer and what not into, mods for the usb, these people have either used iso's etc to mod them, then they have found a way to convert those modded types into usb mods, that people can use offline and online. I wouldn't know myself about the online bit, but I would know about the offline bit as I don't have xbox live. I found a mod either on here or on another site, where is is that generator gametype with no ai's for the usb, the same with maps and other.