You didn't say which capabilities you want for the additional router.
Are wireless N and 1Gbit ports important? If so, you want either a rev. G or rev. I.
The rev. I is $79 new from VZ. Can be found used on ebay for less.
If 802.11g and 100Mbps ports are adequate, then rev. C through rev. F should suffice.
(The rev. F is draft 802.11n, so has limitations as far as 802.11n is concerned).