OP’s usage is fairly modest – web browsing and Office suite (or whatever you use for invoices).
‘Anything’ current will do really.
For the sake of sanity, I’d go 7th Gen i3, 8th will be better as they’re 4 core, but they’re very, very new and will be expensive whilst at the same time making the 7th cheaper.
8GB Ram, but don’t worry if it only comes with 4, RAM is piss easy and cheap to upgrade after the fact.
a Solid State that befits your needs, or an external drive / NAS if you have loadsadata.
All-in-ones are shite, desktops are faster and longer lasting than laptops if you have the space.
Dell are currently very good. Fujitsu are nearly as good, but use more generic internals (easy to fix) – just watch out for lack of VGA output if you’re using an older monitor. HPs are usually robust but are a pain if they break as they use HP only bits.