It all depends on the applications, interface type, bandwidth, and routing requirement: any router that supports BGPv4 can be the minimum CPE connecting to a MPLS PE.
If BGP routing between the PE-CE is needed, any supported BGPv4 IOS version can be used as a CPE. For new router, I would recommend the latest version of IOS 12.3 Service Provider as a minimum; IP Base or IP Voice or IP Security do not support BGP routing.
If interface type is just a T1/E1, no IPSec or VoIP termination on the router, no BGP routing, most of the current or outdated router and image can be used as well. For a brand new purchase, I would recommend a 1841 or 2801 with IP Base for a brand new router. IOS version can be the lastest 12.3, 12.3T, 12.4 or 12.4T.
The Security bundle or Voice bundle with the Service Provider or Advanced Service Provider image on the 2800 series will be a better bargain if Security or Voice features is needed.
For Security, VoIP requirement or interface type other than T1/E1, the minimum recommendation will be different. Please be more specific with the requirements.
Edwin
