Tamron 17-50 f/2.8 AND....
First off, I would always base my lens battery on a very good medium range zoom. You would have that in the 17-50mm f/2.8 Tamron.
Match this medium range zoom up with a very-good tele zoom and you will have a very versatile duo.
My first choice would be the 70-300mm f/4-5.6 IS zoom (because of the convenience of IS). However, this combination will run you about $120 over your stated budget - it will be worth it because you will have IS in the long focal lengths where it really helps out.
My second choice would be the 70-200mm f/4L (this comes in second only because of lack of IS). This combination would hit your $1,000 budget right on the head.
I have not used the Tamron but, I use an almost identical focal length (17-55mm f/2.8 IS) lens and I consider the combination of this focal length (with constant f/2.8 aperture) as an excellent pair with the longer lens. I pair up the 17-55mm with the 70-200mm f/4L IS lens and have owned the 70-200mm f/4L and have used the 70-300mm f/4-5.6 IS lens. I believe that either of these lenses would be a great match with the Tamron 17-50mm f/2.8 lens.
Later on, you could (if funds are a bit short) purchase a used Conon 420ex flash. I have the 420ex and 550ex flashes and consider both of them very good performers. They don't have the bells and whictles of the 430ex or 580ex but, will deliver very good exposure.
Tamron 17-50 + Sigma 18-200 OS + 430 EX - ~$1125




