I like the ESP Ltd 307 series best but unfortunately they aren´t made anymore. Maybe you can get one at evilbay.
Ibanez RG 7s are a little less to my liking but pretty suited fore a nice start.
No, you can play everything you like on them. Originally they were used by jazz player like Georg van Eps in the late 40s for extended chording.
Allthough most 7-string player tune their instrument B-E-A-D-G-B-E or even lower, there´s no rule to do so. You may also use an 11-50 set and add an 08 string for extended lead playing, or even something completely different. Do a little experimenting and check out what works best for you and your music.
They're good for more than just nu-metal - 6 out of the 7 strings are the same as you find on a 6-stringer! How you use the 7th is up to you. The actual guitar (wood etc) and the pickups fitted may seem to aim at nu-metal sometimes (I couldn't help but play some Korn when I tried out a K7), but you can change pickups etc.
That 7th string can either add or subtract variety from your playing, depending on how you intend to use it
I was actually playing one of my 7 string axes the other day and I managed to come up with most of the key riffs for a song I'm writing without going anywhere near the 7th string :eek2: :saeek: :eek5: