If you don't have unobstructed line-of-sight between the two buildings, this is practically impossible.
Yes, you want highly directional antennas,
Even with unobstructed LOS it's asking a lot of WiFi, especially for something that your business depends on. This is particularly true in today's crowded WiFi environment: What worked passably well a few years ago probably won't work so well today, as there are far more WiFi users than there used to be. The old trick of going to channel 1 or 11 doesn't help much any more, either. There are already users on every optimal channel and all the sub-optimal channels too.
And then there are atmospheric conditions to worry about. These bands are open for unlicensed use because at these freqs there is heavy attenuation by water vapor in the air (so they're not useful for long-distance microwave comms... which should tell you something). What works in dry weather will likely stop working when it's humid, and more likely when it rains.
Note that the long distance WiFi challenges that happen around DEFcon are done in the Nevada desert: very dry air and essentially no other WiFi users around.