Hi,
you don’t need to import everything from sage but you have to keep a record of it. You only need to import the trial balances. Have a look here, this should answer most questions:
https://support.quickfile.co.uk/t/importing-a-sage-trial-balance/8935
You also don’t need to continue with your invoice numbers (you can do but you don’t need to). You can start from zero or what ever you want. Maybe this thread is helpful: Customising invoice numbers