For wireless range extension, you have a number of options (excluding WDS and powerline adapters):
- (wired) add a Wireless Access Point to your exisiting network via Ethernet/LAN cable
- (wireless) get a universal repeater
- get a new wireless router ... newer devices may not have an adsl modem though so you might have to use your old one in bridge mode with pppoe on the newer wireless router ... however, there still might be range issues due to interference on the 2.4Ghz band (although the 5GHz band provides many channels to avoid interference, it has poor range due to the fact that high frequencies attenuate faster)
I have some routers used by uk isps including some very nice ones that work great on PTCL with upto 2x2:2 (see wireless n streams) 300Mbps Wireless N (you might have to unlock a few) ... I am (really slowly) trying to accumulate all info at ahmedfarazch.blogspot.com and I have quite a lot of routers to go through! You may find these cheaper than newer ones in most cases!
If you want, you could buy one from me (once you decide upon a method, I can give you a list) ... perhaps a Netgear WNR2000v2 with Tomato Firmware!