I would be very careful if buying from ebay. I would suggest the fs sections, one of the sponsors.
asking if the swamps idm is worth it is opening a can of worms of opinions. I have a swamps IDM with no complaints. Would I do it again on another truck, I could live without it.
yes is makes a noticeable difference. I think beans offers a modded idm for a bit less if I recall. From a performance standpoint, you probably wouldn't notice a difference between the 2.
I think you can get an alliance idm from riffraff. oem from tousley, etc.