I'll try and help as much as possible but it's an awful lot to take in all at one time and a lot to learn but slowly slowly and it will all fall into place.

Generally the Transmitter (TX) has to be compatible with the Receiver (RX), if you think you're going to be in the Hobby long time you're best of getting a good Transmitter now as I've 2 at home that I no longer use and so are now wasted money but add the cost of them and chargers up and I could have just bought the better one staright off and saved myself a couple of hundred pounds. If you're on a budget look at the Spektrum DX6i and if you can afford it look at their DX 7, both come with a Receiver included in the price and the 6i is amasing value for money IMO.

I'm sure from other threads you can use a computer TX wih the SC but I've never done it so not sure how, I wonder if the servo lead plugs don't fit most common receivers (you'll need one compatible to the nex TX) but maybe find a Super Cub thread on the forum and ask in there as I know others have done it.

You don't have to buy a whole new set up if you want more performance as you should get the right set up for the plane 1st off so you don't have to change later.

The mAh on batteries is the amount of charge they hold and how long they'll last per charge but the higher the mAh the heavier the battery is and in an ideal world you want a plane to be as light as possible so you need to find size /weight of plane that's compatible that gives you a reasonable flight time but doesn't make the plane fly like a brick. As an approximate rule most 40" wingspans will ideally use a 1800 mAh and 45" around teh 2200 mAh and will give approx 12 mins flight time per battery.

If that sinks in OK and you want more answers just let me know, if you're looking for a good second plane the Mulitplex MiniMag is an excellent choice as a 2nd plane / aileron trainer.