The Linksys WRT54GL was the ideal platform for OpenWRT/DD-WRT etc a few years ago but these days it's looking a bit weak. The Netgear WNR3500L, that you already have, is far far more powerful:
480MHz CPU, 128MB RAM, 128MB ROM.
Choose one of the firmwares linked to above, say this one, configure captive portal on it. ??.??. Profit! ;)
Steve