First off, welcome to the forum and best of luck finding a V if you choose to buy one.
Here's a few answers for you.
1. There are several software packages available for GM vehicles. The one I prefer is HPTuners www.hptuners.com
. Another popular package is EFI Live.
2. You can use GM Parts Direct as a viable source for GM parts. www.gmpartsdirect.com
-- they also have a failry extensive part catalog you can search through. Another advantage is that you can call the dealer and describe the part to get a part number then purchase from this web site.
3. There are regional clubs where V and other sports car owners get together. I'm sure it's just like any other enthusaist vehicle in that you can find a group meet and hire driving instructors or rent track time. One specific driving course I've heard good things about is Bondurant www.bondurant.com
4. If you don't extensivly modify your vehicle and are operating it under normal conditions I don't see why you wouldn't be able to get parts replaced unders warrenty. I had a differential replaced under warrenty and the dealer listed "excessive burn-outs" on my work order. This was not true but they did cover it under warrenty even though the dealer stated I abused the vehicle. If you use sense in what information you give the dealers service dept you can probably get any breaks at the track covered under your factory warrenty.
5. A CTS-V would be a good option for you to look into. Neither the STS-V or XLR-V are available with a 6 speed manual transmission. The CTS-V and the entire V line-up is not available with all wheel drive.