IMO, a Logitech wheel offers the most flexibility and most newer games will be preconfigured for it.
The Logitech profiler software will allow you to make custom configurations that will automatically be applied whenever the specified game or emulator is launched.
I wouldn't use the Daytona pedals with the A-PAC because they would show up as a separate controller and some games have issues with multiple controllers. You can however, swap out the potentiometers in the Daytona pedals with 10k ones and wire them in place of the original G25 ones.
It just depends on which ones feel better to you. You might be able to finesse the softer G25 pedals a bit better.
I figured out the values of resistors needed to make an interface that would allow you to interface a custom shifter with the G25/G27, but haven't got around to building and testing it yet (busy with home improvement projects....despite the fact that I rent).
Not many games use the clutch pedal, but it's nice to have in a simulator. I prefer to have one in real life driving, but often in games there is so much else going on that I opt to just use paddle shifters to simplify things. The only thing bad about the clutch pedal on the G25/G27 is that the profiler software doesn't allow you to make any changes to it the way you can with the other pedals (company doesn't want to pay someone to update the software to account for the extra pedal). I've only run into an issue with it once, but it still irks me. In the game Fuel, the clutch axis is hard wired to reverse and your vehicle will travel backwards unless the clutch pedal is held halfway down. The game isn't supposed to be compatible with the wheel anyway, but being able to remap the clutch pedal to some other axis would have fixed it.
If I were doing an "ultimate" racer, I'd do triple screens. Even a lot of the old 3D arcade racers have been hacked to work on triple screens.
Nearly all new games that support ffb wheels support triple screens. (There are the oddballs that only support xbox360 controllers)
That doesn't mean you can't start out with the one you have now.
If you're leaning toward the newer game/sim side, check out simvibe.
http://simxperience.com/Products/SimVibe/SimVibeSoftware.aspxI haven't used it, but it has excellent reviews.
I think you'd be making a huge mistake by building something nice before becoming familiar with the games.
Build a sloppy test rig with good ergonomics. Build your PC (get a video card that supports triple monitors as a single super wide resolution, AMD calls it eyefinity, Nvidia calls it Nvidia Surround)
Then play the games and see what you like/don't like, need/don't need.
As far as enclosed vs open. Enclosed would be pretty cool for the immersion factor, but figure on incorporating a good ventilation system to keep you cool.
Sometimes racing for long periods makes you sweat.

EDIT: Check out spacehedgehog's R-Pod:
http://forum.arcadecontrols.com/index.php/topic,138146.msg1426735.html#msg1426735