Hello JW, welcome to the Komando Community. You can’t go wrong with using Dell equipment for your workstations and monitors.
Quickbooks online takes care of storing your information in the cloud. Having a server dedicated to QuickBooks is antiquated. However, if you do prefer having QuickBooks stored locally Dell would also be your best bet.
Regarding your firewall I recommend a Firewalla Gold or a Fortinet Fortigate 60E
If you let me know what kind of apps your users will be running I can make a better recommendation specifically the model for workstations. Generally, you want to say away from i3 processors.