There are definitely sewing machines out there with a larger harp space. I bought an Elna eXcellence 740 used (it was a trade in). It has an 11 inch harp space. It is fairly easy to quilt a king sized quilt, smaller quilts even more so.
I actually quilted an almost king sized quilt with a Singer 15-91. It probably has a harp size similar to the Brother 1500. I divided my batting into thirds for this one. I didn't really enjoy adding the batting back in, but it worked for that size quilt. Most quilts I make are smaller than queen size. I actually loved quilting on this machine (straight stitch, vintage machine). Paid less than $200 for it. I think I need a new bobbin case, after a move, I never could get my tension back right.
With research and patience, I think you can find a very nice machine for less than $1000. I believe I paid about $800 for my Elna. With the quilts I have quilted myself (and thus saved $$ by not paying for someone else to quilt) - my machine has easily paid for itself and will do so many times again over its lifetime.