Yes, the sigma is a good lens for the money. One cannot expect professional result with this chepo lens but for an enthusiast it will serve the purpose.
The Nikon AF-S will be great for macro shots when paired with a good body
a good nikon camers will cost about£600
The sigma 10-20mm EX DC HSM is a great wideangle lens with the Hyper sonic motor you need for Focusing and metering on the DX format. The Nikon 12-24 AF-s DX is quality too - but MUCH more expensive.
YES
i personally love the nikon coolpix s230 it is super user friendly and takes very good pictures!
Canon Powershots are great for macro shots.
As "good" as you want it to be.
The new Nikon AF-S 24-70mm f/2.8 G ED, is a wonderful lens for the job. However if you are on a budget, then you may also want to look at the older Nikon 28-70mm f/2.8 AF-S. Both these are outstanding lenses and the ones that stay on my camera the most... However if both of the Nikon lenses seem expensive, you can always try the Sigma 28-70mm F2.8 EX DG lens. This is a fast and robust lens too. And for your purposes, just fine...
Depending on your camera, lenses are available or not available. Nikon, Canon, Olympus etc., all have very good zoom lenses for their cameras. You will also find good lenses from Tamron, Sigma, and Tokina, that would suite your requirement. However, I am assuming that you have an SLR camera...
Amazon is a good place to start to find a refurbished Nikon camera online and they offer some very good prices for a refurbished Nikon camera online as well
Sigma Pi and Sigma Chi are the only good ones.
A good place to find a digital camera made by Nikon would be Staples. They have tons of choices and I bet they have digital cameras made by Nikon. So go take a quick look.