100 mbit internally is what is going to be predominant in probably 90% of small business uses.
Whatever externally you choose depends upon A.) Cost and B) need
If your company only uses the internet for basic needs, then I imagine the local cable / DSL loop could suffice depending upon size and traffic expectations just fine.
Anything beyond that will require more than just a guesstimate, it’ll require at least a good understanding of whats available in your area.
MPLS, Ethernet over Power Lines, etc etc etc. There’s many choices, but unknown whats available where you live.
Cogent can provide a hefty drop at a decent price up to 100mbit into your network
I agree with above that obtain at least a 100 MB switch or the option to go with a gigabit switch but not really needed in your environment. As per external “ISP” connectivity, I’ve seen small companies get “bundle” deals with their phone provider let’s say “Verizon” in which they would provide a business DSL package at a reduced price. That’s an avenue you may want to review at either looking at your phone provider or cable provider (TV within the office) to bundle their cable internet as well.