Qmail/vpopmail/maildir server conversion to postfix/maildir

I have production server running old setup with qmail/vpopmail/maildir and courier-imap. Qmail is "rotting" and for some time server is not able to process spam filtering and lately I(ve discovered it have problem connecting some of DNS servers and doesnt deliver mails from time to time. I(ve decided it is tome to migrate to new server. I need research about - how to do it in the first place. When it is done I may prolong the contract and let you do the actual migration. I am also not sure if budget is sufficient. It is negotiable.