The Pi is lousy for that unless you get something with an external antenna connector. Back in the 2000s, we set up inter-building links with Cisco Aironet PCMCIA cards (which did not have the EIRP power limits currently enforced for consumer products) at blistering 11Mbps speeds (well, it was fast for the time) and a few years later we did it again at 54Mbps in the 2.4GHz band with OpenWRT devices like the WRT54g, doing 10Km with external directional antennae.
I also built my own 3G range extender with an almost identical off-the-shelf $50 Yagi antenna at a country cottage (2.2 and 2.4GHz are close enough for an antenna to carry both), so I've spent a while pondering those kinds of scenarios.
But addressing your points directly, I would look for OpenWRT-compatible devices with external antenna ports and the ability to do both 2.4 and 5GHz - and most likely try to use 2.4 for long-distance point-to-point links with Yagi antennas.
Hitting the bitrates you mention seems perfectly doable, although I must point out that the antennas are conspicuous (about the length of two Pringles cans). The challenge I found is having router hardware that can pin SSIDs to separate radios, because most of the modern stuff aims for diversity and prioritizes throughput.
I also built my own 3G range extender with an almost identical off-the-shelf $50 Yagi antenna at a country cottage (2.2 and 2.4GHz are close enough for an antenna to carry both), so I've spent a while pondering those kinds of scenarios.
But addressing your points directly, I would look for OpenWRT-compatible devices with external antenna ports and the ability to do both 2.4 and 5GHz - and most likely try to use 2.4 for long-distance point-to-point links with Yagi antennas.
Hitting the bitrates you mention seems perfectly doable, although I must point out that the antennas are conspicuous (about the length of two Pringles cans). The challenge I found is having router hardware that can pin SSIDs to separate radios, because most of the modern stuff aims for diversity and prioritizes throughput.