Although I Love steam and the way it works (Especially the social side of things) I don't think steam is a good idea, Their client wont let you play your games without internet.(you have to be online to activate offline mode) So you would only be targeting an audience that has a (almost) constant connection. and although all games get cracked eventually, out of all the games I have seen. Steam games are usually cracked the fastest. I haven't downloaded a cracked game since minecraft survival test (Which I bought 3 accounts before It reached 1.3 Beta), but I have watched development on them and the average steam game gets maybe 1 day before it gets cracked. I haven't really researched other clients though.
You only have to be online once to activate offline mode, you just have to make sure you activate it at least once. And seeing as you also have to be online at least once to download it, it kinda evens out. Steam is a form of DRM. You can't just pack up most steam games and send them to someone else for them to work. A good example of cracked games would be gog.com, where you CAN do that. You just posted so many contradictions I'm not sure how to respond.
um.. I'm pretty sure you don't have to get the steam keys. it looks to me like you'll be able to download the game outside of steam.. the steam keys are just there for people who want to use steam.. I think Terraria did that too, but I'm not sure.