Please take a look these points before you buy a colocation service.
1.) Distance (try to get coloc service closer places) The more close you get this service faster responses you will get.
2.) The Price, of cource itshould not be expensive in terms of same bantwidth.
3.) Datacenter conditions and facilities. Make sure the datacenter is stable and secure.
(my own artticle) no copy!
4.) Stability, some DCs are notvery stable so make sure working with the ones %100 uptime.
5.) If you host webmaster forum make sure you buy a firewall and attach the colocation
(My own article)