* A decent Internet connection with SLA, domain & webhosting (though we use ourselves for all this as we're in this space)

* Xero for accounting

* Regrettably, M365 for email/contacts/calendaring/office apps/file sharing cos we need groupware of some sort and gsuite is.. no.

Sector specific;
* some sort of e-commerce front for the self-serve services like domain registrations (we currently use WHMCS for this)
* Halo (PSA/CRM/Helpdesk/etc) 

 
and;

* some sort of backup software/service configured so as to take your data somewhere it is not trivial for some ransomware weenie to erase it