Glass Glazing Requirement!

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.

Triple glazing is usually touted as the best for energy reasons.
Sound reduction in triples can be better than doubles, but it's not dramatic. Asymmetric glazing (different thicknesses) can give triples an edge by blocking (attenuating) different frequencies, but these are special features not on the average triple. The larger air gap of a double is a better sound insulator.

Your best comparison, insulation-wise, is to compare the R-value / U-factor of 2 units. The U factor is nothing more than the reciprocal of the R value. A window with a R 10 rating will be a U 0.10 (a great window currently.

Triple glazing is the darling of superinsulated homes and has a price to match. They typically only make sense in colder climates where heating costs would justify the expense. Windows must be paired other areas of energy conservation: air sealing, ceiling insulation. An energy audit that does a blower door test, paired with a thermo imaging analysis, should be done before upgrading to triple glazing. Some local utilities have discounted energy audits.