Assuming you don't want to get into large format, tops for landscapes and large enlargements is a 6 x 9. There are some great deals out there nowadays. I picked up a Horseman VR that was formerly a police unit, for $750 with lens, case and some accessories. The entire set would have run some $3-4k new. There's a unit right now with multiple lenses (ebay item 3873590578).
Mike