I'd definitely check out Halo 2. The modding community was massive. In fact, modding Halo 2 was what introduced me to programming :)
The maps have a very basic encryption(checksum). Map signers are all over the internet if you don't feel like doing the work yourself. There's also many high level tools to play with the maps. I remember taking an a vehicle from one level and placing it in another was a trivial process. Youtube has a plethora of videos where modders show off what they could do
I was referencing the xbox version of the game. If I remember correctly, the pc version of the game actually allowed cheaters, which takes all the fun out of it IMO
Im sure you could find an old xbox and the game for less than $90AUD. Or, you could buy an xbox, flash the disk drive, download a halo .iso and burn it to a verbatim disk. Thats what I usually do
The maps have a very basic encryption(checksum). Map signers are all over the internet if you don't feel like doing the work yourself. There's also many high level tools to play with the maps. I remember taking an a vehicle from one level and placing it in another was a trivial process. Youtube has a plethora of videos where modders show off what they could do