The original Mass Effect will be pretty painful to play if you've already played ME2. I was given that very advice by many, many people, and I disregarded it. I thought it'd just be different, maybe even not as good - but that simply wasn't true. The plot is stronger (it has the advantage of being the first of a trilogy) and it's better paced once you clear the gelatinously slow first quarter of the game, but that's it.
Virtually every aspect of the gameplay screams "unrefined" in comparison with its sequel. Core shooting and cover mechanics are unpolished, the inventory is an impenetrable mess and even basic movement feels pretty bad.
That being said, it's still an above average game and Mass Effect 2 is a much more interesting game to play with an imported file. Plus, with ME3 on the horizon, having a file that has been carried through both previous games to import will no doubt be an improved experience.
As for Bioshock, it's a great game and unique experience, but like the original Mass Effect, its mechanics and design are beginning to show their age. Since everyone and their mother already knows the big plot twist, it might not be worth the cash any more. If you have a console and you're curious to see what all the fuss is about, I'd say pick it up pre-owned for a console instead. Not because it's a better experience on a console or anything, it's just not particularly worse and will be much cheaper.
When it comes to DLC, you'll get the best deal (in terms of play time offered) from DA:O:Awakening, though it doesn't do much different from the main game and the other DA:O DLC is pretty weak. Mass Effect 2 DLC is a blast to play, especially LotSB and Kazumi's loyalty mission, but they're pretty low on content for the price. In fact, you could probably finish both additional characters' loyalty missions along with LotSB and Project Overlord in a single sitting if you have a day to waste.
All in all, I'd say if you plan on picking up ME3 without a doubt, then get ME1 and scrape together the extra cash for ME2's DLC. If you don't care much about ME3 but you felt like DA:O needed to be 20+ hours longer, get Awakening. Otherwise, Bioshock it is.