Hi! I want to cover a long house of 30 meters with wifi using my 2 x EX5601-T1 and 1 NWA55AXE (all flashed with openwrt). The nwa55axe will be mounted on the outside of the top wall.
Would my hardware be enough of should I buy into Ubiquiti hardware 
?
That depends on where you place those three APs, the interior walls (and materials), if you can interconnect them by ethernet (or need a wireless backhaul) and if there's a 2nd/ 3rd storey to consider - and what kind of minimum throughput you expect. The hardware vendor/ firmware shouldn't make much of a difference though.
This is going to depend on many factors. Have you done any kind of survey? I would set some temporary devices and then measure signal strength with a Wifi scanner app on your phone. take node of problem areas and channel usage. What performance are you wanting? 2.4Ghz will go a lot farther but it has way less channels and is slower. 5Ghz is fast but it has a limited range.
Paying doesn't usually fix bad planning
Should suffice, make sure you have wired backhaul
