Lots and lots of questions! Well the 15-85mm is pretty wide at 15mm so if you shoot landscapes this might be wide enough for you and not have to go to the expense of buying a dedicated wideangle lens. If you are thinking of go down the 3rd party lens route i wouldn't go down the sigma 24-70mm f2.8 route at £600 I would get the Tamron 28-75mm f2.8 which is sharper and only £300-£350. Tamron also make a very good kit equiv lens, the 17-50mm f2.8 which is much sharper and fast than the kit lens and only costs £240 on amazon.
The pro1 uv filters have a low profile rim to stop vignette which is a problem on wideangles. £60-£70 is the prices for polarizers and neutral density filters. You can get a hoya pro 1 uv filter for about £20 on amazon.
If i had £2000 to spend it would look a bit like this
Nikon D5100 body £525
Tokina 11-16mm f2.8 £500 (Sigma 10-20mm £400 the Tokina is worth the extra £100)
Tamron 17-50mm F2.8 £270
Nikon 70-300mm VR £365
Nikon 50mm f1.4
Giottos MTL9351B & 5001 head £85
yongnuo yn465 speedlight £40
Woops i've spent to much and you will need a bag and a few filters too!
