Your best bet is to try and find a reconditioned 837 (think ~ R 2500), but that will only do 2 lines (internal ADSL + cheap external ADSL modem on one Ethernet port).
Next step up would be the 877 Adv IP ( ~ R4500+) which could do 4 lines ( 1 internal modem + 3 external cheap ADSL modems on Ethernet ports ).
what seriers are these routers? i have found a site that sells new and 2nd hand