That unlimited Three deal is also available from Smarty who are doing it on a monthly basis for £25.
You can check your coverage on Three's maps, or as I did, put a PayG sim in your phone and double check it and test download speeds.
I use a dual sim Teltonika RUT950 4G router for my home broadband which was about £180. As the signal out here in the boonies is rather weak, I also bought an outdoor antenna (£60). Both fetch good second hand prices on eBay so you can get quite a bit back once you've finished with your hardware.
I use a Three unlimited sim, speed is typically 12-15mbps. Three's speeds may well be better in other areas, I really do live out in the sticks!!
Latency is on the high side so 4G is probably not the answer if you're gaming or running apps that would be negatively affected by high latency.
Also.. an unlimited sim from EE is available via Virgin Media if you are already a Virgin Media customer.
EE should (in theory) be the fastest of all the mobile providers.
There's no throttling or download limit on the Three sim.
I regularly use the sort of download amounts you are talking about.