Hi,
There are a wide range of manufacturers, Ubiquiti is known as the most affordable option. Many people find that Cambium is more reliable, but there are others as well, some of which Marc has mentioned. For point-to-multipoint setups you will generally have the easiest time buying the base unit and all the CPE's (customer premise equipment) from the same manufacturer. I've also heard good things about IgniteNet, although they are not as well known as some of the others.
I think it's not a great idea to try to flash your long range transmitters, as the manufacturer firmware often has great features and is very heavily tested. Instead, have the equipment with the custom firmware be a standardized model of linux router. We have a bunch of MyNnet n600's at Sudomesh, although we also still flash the long range hardware. Some manufacturers also have their own physical layer protocols which work better for specific use cases than WiFi. I wish that all manufacturers committed all firmware improvements to OpenWRT, but that's not what they do and you shouldn't let it slow down your network.
I would also recommend you seek out advice from wireless ISP operators for advice in addition to community mesh networks. Wireless ISPs are active all over the world and serve many millions of users. The vast majority of wireless equipment out there is operated by wireless ISPs. I can add you to a group on Facebook (called "WISP Talk") where you can ask these questions to a few hundred independent American wireless ISPs. Message me off list to coordinate. I'm not a huge fan of Facebook, but that's where this group is.