What about one of the Jukis? The 2010Q might work for you. It's a high speed straight stitch machine. Here's a page with info on it:
http://www.jukihome.com/products/quilt_tl2010q.html
I think you might like it because it has a large harp and feed dogs. You can search here on QB for more posts about it.
I don't have this machine (I have a Sweet Sixteen sitdown) but I do have a Juki sewing machine and I love it. I've never had to adjust anything on the machine, including tension, and it sews like a dream.
Hope this helps!
Lynda