Looking for help with a server setup for my small tax business

I'm going to engage a local business to help with my setup, but wanted to see if I could get a few pointers, tips, etc before I end up either not getting enough server resources or getting way too much.  Any help, feedback, etc is much appreciated.

Here's what I have - what would you recommend that I need?

1. I have a tax software that will need access for 5-6 people, not devices, so I know I'd be looking at the User CALs vs Device CALs.
2. Sometimes they will need remote access to the software.
3. Currently our email is sort of scattered, gmail, outlook, etc - I want to give everyone their own "bob@taxservices.com" email address.  

So my general questions would be

1. What type of Windows Server should I go with?
2. How many servers would I need to run just the tax software and our email? 
3. Any recommendations on a particular server brand vs another?
4. What am I missing/not thinking about?