I am very interested in the SG-2220, as it is a certified piece of hardware with a solid looking set of specs.

Pending on your art of usage and the not available extra packets, it would be really nice for you.

As far as I understand, if I buy the unit with an mSATA SSD installed pfSense will be installed onto it, but if I buy the unit without the SSD, pfSense will be installed on the on-board flash.  Is this correct?

Yes this might be correct, but I in your situation would more send a email address with a pre-sales question
to the pfSense -store itself.

If I were to buy the unit without the SSD, can I insall an SSD in the future without voiding the warranty? How would I move the installation over?

You might be installing it on the eMMC storage or a mSATA, this did not avoid the warranty as I see it right
but again those questions should be asked not in a forum but more send directly to the pfSense store via mail.

Any help/direction that anyone could provide would be appreciated.

Get this device without a mSATA and use the eMMC storage and if you decide yourself later then
to get a mSATA you only have to change the boot order in the BIOS thats it, and perhaps on top
copying the config backup over to the new installation, not more and not less.

A custom ADI image, matching to your SG-2220 unit, will be available to download after the registration
of your SG-2220 unit.

My internet connection, if it matters is 76 down/19 up, but I am getting nowhere near that right now because the way my network is set up.

The SG-2220 will be handle it with ease.

Why would you not want to get it with an SSD?

Just evaluating my options right now.  I am asking myself if I need it.  I have been running pfSense quite happily on my 4GB VM (disk is currently reading at 16% full), so it sounds as though the 4GB flash would be plenty, but I don't want to paint myself into a corner.

Finally, the mSATA drives offered don't seem like terribly good value (£47 for 60GB, where I could pay £34 for 120GB if I bought aftermarket).