Considering buying WN3000RP for my house

As you seem to recognise, for a repeater to work well it needs to be in something of a ‘compromise’ position, that is it needs to be close enough to the base-station to receive a reasonably good signal while at the same time far enough away from the base-station such that it provides wireless coverage to distant clients. As a quick and crude test before you spend any cash, you could move the router to where you plan to place the repeater, and see if a PC at the distant location can receive its signal (you don’t need the router to be connected to the Internet for that).

Have you considered using Powerline products? These can use your mains wiring as a way of sending data around the house, one Powerline device close to the router and connected to it by Ethernet, and the other Powerline device can be anywhere on the same mains circuit. Powerline products can provide a WiFi connection at the distant location.