*any* glass made by the float glass process will have the same level of flatness (>95% of today’s colorless glass). However, thicker is better and I would see 5mm as the absolute minimum. When glass slabs get bigger/heavier their shards pose a real risk in case they crack. Therefore, 5 or 8mm tempered glass is what you should be looking for. If you can’t find tempered glass, cover the back with UV-stabilized (all weather) tape: this will hold the shards together and you don’t get any exposed edges.
Non-Standard Miter Slots - This one is a downer. One of the primary advantages of having a table saw is access to jigs that expand the saws functionality. This is a major issue if you plan on buying after market jigs. Given that we are limiting the cost of this buildout to $500, I am guessing that after market jigs are probably low on the priority list. Your going to want jigs once you start researching what they enable you to do, my advice is to build your own - there are plenty of plans online.
Matthias Wandel comes out with some amazing modifications for tools and techniques. This one however is incredibly simple and only requires a few scraps of wood and a pair of clamps to turn a simple circular saw into a custom width dado maker. You can spend a lot on special dado blades for table saws or router with a straight bit and some guides, but this method is simpler and the results are very close to the same. Very little is sacrificed by using this frugal dado making method.
Ebates.com - Ebates is something I'll check after I've already found a deal that I'm going to bite on. The rewards aren't usually good enough to compel me to purchase on their own. But if you get an extra 3-5% cash back over your entire $250 budget, it can add up to an extra hand tool for doing nothing more than using their link to an online store.
A layout square, or combination square, comes in 6” and 12” sizes. Most woodworkers use the 6” model, simply because it’s easiest to carry around. Also, most of the stock you’ll use will be no bigger than 6” wide, so 12” is overkill. The layout square is a triangle that you can use to mark square cuts on stock. Once you measure the length of the cut, you line up the layout square with the edge of the board. The short side will give you a straight, square cut across the end grain. You can also measure off angles with the layout square. This helps when you’re trying to measure for a bevel on a table saw, or marking a cut for a miter saw. You can even use your layout square to determine an existing angle. Just be sure to buy one made of metal. The plastic ones are not only fragile, but they also can warp, making them pretty useless.