Your westell 7500 is a wireless b/g router so I wouldn't spend too much on a high end wireless n extender. Since your max speed will not exceed 54Mbps (g) any extender you find will be limited by that.
Check this one out: http://www.newegg.com/Product/Product.aspx?Item=N82E16833704081
TPLinks are pretty good, this will do the job nicely, you will also have up to 150Mbps if you upgrade your router later on.