D&D Collection is awesome because Shadow Over Mystara is my favorite 2D beat-em-up game!

With that said, I think it's worth picking up. However, it's kinda pricey.
Only having 2P kinda sucks, but I usually play solo anyways. IIRC, the Saturn version is based off the bug-fixed arcade but I'm not sure if there are actually any differences between the Saturn and that arcade version. Unfortunately, the bugs in the arcade made the game even better... except for the invincibility/all items bug. That ruined the game entirely... and it would crash the machine.
Unlimited continues kinda ruins the fun too. I can 1cc with every character, so I don't use continues anyway... that's the challenge.

Also, the game keeps track of the items you've found. Don't remember what happens when you get them all, but I don't think it's anything special. However, there's no way to get all the items on one play anyway.
Tower of Doom is alright. I think it's harder than SoM, but that's mostly because the controls suck IMO.
As for the arcade boards, I remember seeing a number of them (especially Tower of Doom) in Akihabara the last time I was there (Sept 06). I was actually specifically looking for the SoM board (about ~$150... maybe a little less). Maybe one of the Tokyo forum members could help you out.