drupal 7 to backdrop migration

I am considering migrating my drupal 7 website to backdrop. I want someone to do the complete migration. I have read some of the documentation and have some questions. My website (I think is reasonably simple) but it contains some things which may be complicated and which need to be migrated. Excuse the way I explain things. It has 1 custom module (1 module file and multiple php files) - can a custom module be migrated without re-writing all the code? But the custom fields have been added directly to a couple of core tables e.g. the users table contains many additional custom fields. In this case can that table be migrated and can the data be migrated? It also uses the forum module - can that be migrated. It uses commerce sagepay - if that cannot be migrated it is not a big deal - any commerce module which works will do. Ideally I'd like the person who does the migration to have experience of these particular issues.