Hanging 20“ x 16” glass pane

The T-square is just that - a T-square for making sure that the glass is cut square, unless it has an integral glass cutter of course.

The tool they use is a glass cutter. It will have a diamond wheel or tip and scores the glass where it needs to be cut. Then a sharp tap will (hopefully) produce a clean break.

They should be easy to pick up at any hardware store. You might have more difficulty getting hold of a large enough T-square, but for a cold-frame you don't necessarily have to be 100% accurate.

Glass can be quite expensive. It would be worth getting a quote for buying the glass you would need and then compare to the cost of the tools you need to buy and your time.

However if (as chris points out in his comment) the glass is tempered then you won't be able to cut it. It will simply shatter if you try.

Glazing manufacturers have spent decades perfecting a spacer system that does not eventually leak and condense. They still haven't completely perfected it, though they are much better than 20 years ago. There is no way you're going to put together a system that does not eventually leak, especially with a vacuum. You'll have a better chance with inert gas, but I'm not convinced even professional argon filled glazing stays argon filled years later. It's not that I doubt your fabrication skills, it's just that it's a very difficult problem to address.

For all the time and effort spent assembling a window system, it's worth buying professionally built glazing units. If you want to save some money, forgo the argon filled and simply get dual glazed units with a low-E coating. IMO, these offer decent thermal performance and good value.