Sigma 17-70mm

[Beav]

Hello. I currently own a 10-20mm sigma lens, 18-55mm kit lens and a 55-250mm zoom lens. I have a 400D body and the 18-55mm lens to sell. Im hoping to get enough from these to buy a sigma 17-70mm in order to cover the kit lens range. In the future ide like a 70-200 which is why im thinking the 17-70mm will be a good range.

A few questions:

1. Should I get enough for a 400D + 18-55mm kit lens to buy a 17-70mm sigma (second hand)

2. Is the lens worth getting?

Thanks