If you don't have employees, you can consider QBSE. Otherwise, you should start from QBO Simple Start and upgrade your plan later. QBSE and QBO are two different products, and no 3rd party apps support QBSE.
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QuickBooks Self-Employed and QuickBooks Online are designed for specific types of businesses. QBSE is designed for freelancers while QBO is best used for small businesses in general.
You need QuickBooks Online if you need the following features:
Payroll
Bills and Bill Payments
Quotes and invoicing
Track VAT
Multicurrency
Customized reports
Project profitability and budgeting
Cash and accrual accounting
Manage Construction Industry Scheme obligations
On the other hand, choose QuickBooks Self-Employed if you need to:
Track and claim allowable expenses
Manage business income and expenses
Prepare Self Assessment
Import bank transactions automatically
Invoicing
You can see more details about their similarities and differences in these links: