IMHO - there are 2 ways to look at this.
1. If you HAVE to get a printer (you didn't say what kind of volume) and you want to do it on the cheap, I would guess an old JV3 would be perfect if it is in good working order. They are built like tanks, can handle the heavy weights easily and will print all day without missing a beat and handle everything up to 5' baner. However, I would stress that having a printer is not the entire solution. Realize you will have to have big room and big tables (5x10-12' minimum) Even so, you'll get the orders for 15', 20, etc. and will need to be able to work on those efficiently in your shop. Add in a grommet machine, hemming tape and/or a sewing machine and you'll start to realize there is more to it than just the printer.
2. If it were me, and ALL I was going to do was banners - I would sub it out to someone like Signs365 or another wholesaler and know that I'm getting a good deal at $1.00-$1.49/sq ft. and mark it up to what my market will bear. NO PRINTER TO REPAIR, NO SEWING, NO GROMMETS, NO MESS and NO LARGE TABLES AND SPACE NEEDED!!!