Out of the 3, style-wise, I like the Vengeance. I have never heard of the brand so I cannot give feedback on quality.
Only if you want to although it is recommended. If you swap out the springs to lower, it is better to upgrade the shocks too. The change in the springs will probably decrease the life of the OEM shocks. You can wait until the shocks fail and then replace. You will just have to pay for installation twice (once for springs install and later for shocks install).
Any change is a change. Only you can decide if it is worth it. It was to me. :)
No matter what setup you decide, the wheel gap will NOT change UNLESS you change the overall wheel diameter. If you get the appropriate tires, ground clearance and fender gap should stay the same. It will only LOOK lifted because of the more metal/less rubber aspect.
I would go for a staggered setup. I would go with 8.5 or 9.0 front and 10 or 10.5 rear. With these setups, offsets are important. The three wheels you listed did not show offset info so you are taking their word that things will fit. It also comes down to how aggressive/flushed you want the setup. Do you want bolt on or you are willing to take a few steps to have it more flushed?
Best you can afford for the conditions you plan to drive it. :) Tires come into play for going AND stopping.