If you go that route make sure everything is presented in a date sorted manner in relevant bundles not a couple of shoe boxes with the papers just crammed in. If an accountant accepted such a mess you'd be paying through the nose for him to sort it out before even starting to do the accounts.
But a simple cash accounting book is more than adequate for the self employed, though doing the maths is can be a bit of a chore and error prone. That is why I keep my business data in a spreadsheet on the computer it does the maths far quicker and more accurately than I could. It also allows simple tracking of creditors and debitors.