What is the latest macOS supported by a 2013 Mac Pro, and what type of SSD can I install?

<< what is the latest macOS a 2013 Mac Pro will run -- supported by apple? >>

MacOS 12 Monterey.

<< what type of SSD can i put in there? >>

Owc/MacSales is the only vendor who decided to develop an aftermarket boot SSD. But do not discard the old one, you will need it for some required firmware upsdfates.

<< where is that model on the year (entry or top of line). >>

vs today's requirements, it stinks.

But PROVIDED everything is working perfectly (the graphics cards tend to die) that is a top processor and a middle graphics setup.

if you look up the processing power benchmarks, ANY Apple-silicon Mac out-computes it.

I would not recommend paying money for 2013 model.

A 2013 Mac Pro can only run macOS 12 Monterey which is no longer supported by new updates.

see > Mac Pro (Late 2013) - Technical Specifications - Apple Support

and > Identify your Mac Pro model - Apple Support

That Mac Pro model is middling. 6-core with mid-range clock. The Mac Pro 2013 got very little in terms of hardware upgrades, too. (tech spec details)

If you’re looking for cheap, I see a Mac mini M1 available for USD$250, and it’ll handily outperform the trashcan.

And it’ll run macOS 26; current.

Handily outperform? 2.7 GHz 12-core Xeon 675 single and 5124 multicore versus 3.195 GHz 8-core M1 at 2377 and 8204. Intel Xeon tends to be toasty warm, too. (And your Mac Pro 2013 here is nowhere near as fast.)

As for old Macs for running old macOS versions, sure, whatever meets your needs. You can boot most (all?) off external storage with them, too.

Models running macOS 10.13 and earlier will be having issues with secure network connectivity. Variously newer versions than that can have problems, too.

As a resource for these sorts of questions, download the MacTracker app from the app stores.