As you've already been told the only Mac that can meet both of your conditions is a Mac Pro. I'm sure the new Mac Pro is way out of your financial league, i.e. basic $6000 w/o monitor or wheels to move it around on. You might find a used or refurbished older Mac Pro but you're getting an older machine and will also need a monitor.
Also a 27" iMac can have it's RAM be user upgraded at less than ½ the price that Apple charges. However, you're stuck with the hard drive that you order.
I ordered a 500 GB SSD in my 27" iMc with 8 GB of RAM. I added another 8 GB for about $79. I have 4 TB of external HDs in two OWC drive docks and a conventional 6 TB EHD for Time machine:

It might not meet some people's esthetics but it's so easy to switch out SSDs for other uses like running a beta system, etc. Boot time from my internal SSD is abut 25-30 seconds. From a USB 3 SSD about 45-50 seconds.
So don't rule out external drives for additional storage and use.
