I'd probably look at ubiquiti. I would call them and see how much they will help you with actually surveying your needs and developing a real implementation plan. Unfortunately they generally sell either 2.4 or 5GHz radios for their outdoor line, not dual-band. Looks like the exception is the UniFi UAP AC Outdoor but it is Omni-only with no sectoral option.
I have never liked EnGenius much, though my opinion was developed years ago.
With that density something like Ruckus is probably out of the question.
https://www.cdw.com/shop/products/ZONEFLEX-T710S-120X30-DEG-OUT-DB-AP/4242939.aspx
Only US$3000 per unit, though you would probably be amazed at how well something like that pole-mounted angled 15-20 degrees downward would cover an open area.
The T301S (120deg) and T301n (30 deg) are about half that. You would probably find that the 120deg unit with BeamFlex would do just fine.
http://a030f85c1e25003d7609-b98377aee968aad08453374eb1df3398.r40.cf2.rackcdn.com/datasheets/ds-t301-unleashed.pdf
Looking at those BeamFlex antenna arrays is almost arousing.
If you're not afraid of used and don't need AC, you might be able to find some good deals on used gear on ebay.