I have an Epson 4990, so the V700 is a step up. I am satisfied with the quality of the scans for 645. Would I like a better scanner for this size... absolutely, but I can't justify the cost of a scanner that would do a better job.
You shouldn't have a problem with 16x20 from mf negs. I get very good 11x14 from 35mm slides. Just remember it needs sharpening I like the smart sharpen in ps
should you ? Well I can't answer that, I can say the V700 is a decent scanner. Its good for about 2400dpi
You say you're scanning 6x9 ... well 2000dpi will give you at least good 8000pixels along the 9cm side. Printing at 300dpi will give you 28 inches wide. I'd say that's enough margin of error to fit within your 16inch.
How much scanned detail do you want to throw away when you downscale to print to 16 inches?
It's the best option you have right now without picking up a much more expensive Nikon or Minolta medium format scanner. However, the med format film holder that comes with the epson v700 or v750 is famously bad. Check out www.betterscaning.com for a better film holder. The difference between the V700 and the V750 is that the V750 has the upgraded software (which you should be able to upgrade separately anyway) and the V750 comes with the epson wet mount holder, but the betterscanning wet mount film holder is better anyway. I think that the V700 with the betterscanning film holder is the best way to go without spending big bucks. When you do encounter that very special negative that must be perfect have it professionally drum scanned by the folks at eiger studios.
While we're on relatively the same topic, has anyone scanned up to 20x20 with the V700? Also, when it comes to resolution and printing does it matter beyond 300dpi, especially at a size like 20x20?