Ok well if I were you I’d buy solspace user and template the front end exactly how you want, and just have members log in there instead of the CP. Id also ask the client if they really want/need private messaging - seems a bit pointless unless your members are complete strangers and dont want to give out their email. I bet you could ‘hack’ your own private messaging functionality just using safecracker. Where you have a channel called ‘messages’ that you relate to a user in a custom field…
Rest assured you’re not the only one that wants to get rid of the icq/aol fields!