I have thought and looked for years. Just a few comments
1. Make sure you can do reverse, many industrials only go forward.
2. Will you want to use the machine for anything other than peicing? Check to see if there are feet that are compatable for free motion, many do not ! Feed dogs are not able to be lowered on many. Yes you can cover them but , the covering creates a drag as you move the quilt around. Tape only lasts a short time.
3. Parts for industrials can be expensive and difficult to get , that's why many are for sale. check on service availability.
I saw a longarm type that was mounted on a table at a quilt show ... I just can not remember the name . It was subtaintally less money than a traditional longarm with frame. it could also be used for a straight stitch with regular feet.
It was at International quilt show Chicago last year, too bad I just tossed out my program with all the vendors listed. Maybe someone else here knows .
This is a link to the type , but was not the exact machine , I will keep looking as it does not appear that this can do regular stitiching.
http://www.allbrands.com/products/abp24354-0073.html