1. It wouldn't be too much of an issue really. I'm a bit of a monitor nut myself but I wouldn't mind using different screens for the sides, there may be some colour/brightness difference, but nothing a bit of tweaking won't make acceptable.
3. With programs like FL Studio etc that'll work fine. When you're in desktop mode (as in not running a fullscreen application or game) you can place windows anywhere on the three monitors. It'll be largely beneficial too for music production
4. The triple head systems will really only be in use for full screen applications, so either will provide the same functionality in a desktop scenario. SoftTH will support that res, with a Geforce 8 or better card (which I see you have). With a single card, there will be negligible/no difference in performance between SoftTH and TH2Go.
5. Not too sure but you must remember to get the digital version to support that resolution.
6. LFS will be fine with those specs, I have a similar setup (slightly older, 8800GTS 320 and a E6600) and I got good framerates. I don't know about iRacing though.
If you want a guaranteed solution that'll work across all games go for the TH2Go, if it's just LFS (and iRacing if it's supported) just go for SoftTH.