It depends what you want to use it for. 12.4 is a later version than 12.3 (funnily enough) but I doubt in a domestic ADSL installation you could find a difference.
You should not be seeing different costs for 12.3 / 12.4...
If you buy a second hand router you're not licensed to run IOS anyway unless you buy the software separately from a Cisco reseller.
Comms is hard