The D90 is a cracking camera but not water sealed, so need to be careful if humidity is too high. Not sure the lens selection is the best, as the 18-105mm VR is better. Not sure the Tamron 70-300mm is good enough for this camera.
My choice would be a Nikon 70-300mm F4.5-5.6 VR and Warehouse Express do it for £999 with the D90 as a bundle.
http://www.warehouseexpress.com/buy-...-lens/p1028015
You can combine this with a Sigma 17-70mm F2.8-4 for £209.99 from Warehouse Express. Buy two SanDisk 4GB Extreme 30MB/Sec SDHC Card at £57.98 and forget about the filters. OK it's a bit more than you wanted to spend but worth it for the quality you get. With VR on the 70-300mm you won't need a tripod so saving a bit of weight. The Sigma 17-70mm get rave reviews from lots of people who own it.
As for waterproof bags I can recommend the LowPro DryZone 100 at £214, as I have one and used it in all weathers and my stuff remains dry in the inner bag.
Just add, you could go for a secondhand D200 camera which is weather sealed and a semi-pro camera and as long as you keep the ISO below 800 will serve you well. I brought mine new when it was launched and still use it along-side my D3, and it performed faultlessly in heavy rain. Greys of Westministers have them from £384 to £449 and the D90 was based on this camera.
http://www.graysofwestminster.co.uk/...3e346e26a0680f