depends on your requirement and budget

a wifi extender (a.k.a 'wifi repeater') will halve your bandwidth. for example if your connection is 30Mbps and when connected to your main router you can get that speed - if you are connected to your wifi extender. you can only hit 15Mbps. another catch is it is not that stable if the main router uses a different chipset or is of a different brand (e.g. broadcom and atheros). not sure if the 'universal repeater' function on some wifi access points these days have overcome that stability problem.



a powerline wifi extender however is different. (homeplug.. generally there are 2 types, one with wifi built in (e.g. tplink TL-WPA8630) on the plug and one without wifi, only LAN port (e.g. (tplink TL-PA4010P)
...in theory you can get the same speed as when you are connected to your main router (if you check the reviews on lazada, some users said they managed to get full speed like connecting a cable directly to the router) it depends on the wiring of the house as well as the powerline product itself.

a more expensive router will of course make the coverage stronger at those areas you mentioned (upstairs and back of the house)
and this expensive router will definitely handle your torrent downloads and whatnot better than your old dlink