Foam floor tiles from most of the manufacturers are very similar with one side being smooth and another having some sort of the ribs or dimples. VRC field has them smooth side up. It might be hard to describe the difference in texture and firmness of the tiles, but once you see and touch official tiles, it will be very easy for you to recognize if specific tiles are compatible. I think you could find such tiles in any of major hardware stores.
Also, most of the beginner teams don’t have any autonomous until January - February time frame, so you will have plenty of time to get your field setup.
It is more like $250, because with Super Kit you don’t get a second battery, which I think is very important to have. If you can afford a total budget of $2500 - $3000, then I would suggest to buy Super Kit.
But if you want to try to stay under $2000 then, in my opinion, Starter Kit plus some extra parts is a better choice because it lets you concentrate your funds on the items that are important. Also Starter Kit has some nice pre-cut structural pieces that are not included with larger kits.
Later in the season, you may want to buy additional items that are not included in either of the kits (like longer standoffs, limit switches, potentiometers, or ultrasonic sensors) and you will need those saved funds.
Additional, highly recommended, items like antistatic spray and magnetic micro-usb programming cables (1), (2) will quickly eat into your reserve funds.
You also need to plan to attend one or two early competitions as a spectator and talk to mentors on some more established teams to give you tips and/or let your kids follow their students for a couple of hours.
On the limited budget, you really want your money to have the largest impact, and that will ensure that your $100 registration fee for the first competition is not wasted if your kids are not familiar with the process.