You have many things to consider. First why are you considering a custom CRM solution? Is your current CRM not doing everything you want? Does the new version not do what you want? Is there another product available that does what you want? A custom solution will take time to develop and most likely cost more than a packaged solution. But hopefully you will end up with a product that far exceeds what you could have purchased and can be readily modified as your needs change.
If you are going to do a custom solution you will need to totaly re-evaluate how you do things to make sure you are doing things the way you want to do them (the way they should be done) and not just do them because that is the way it has always been done.