RE: 2.4 GHz... Will one transmitter work with other brand recievers?
There is nothing wrong with the Spektrum DX5, if all you need is 5 channels. At some point in time you will probably want more channels. As you move up to radios that have more channels, you also get more of a variety of programming features.
In 1978, I bought my first RC Radio. It was a 6ch Futaba. It had no Servo Reversing, no Mixing switches, no Dual Rates or Expo. When you set up an aircraft, you had to think about which way the servo moved and be sure to put the Servo Arm on the correct side. There were quite a few Mechanical Adjustments you had to make as there were only Trim Levers on the Tx. If you used your Tx for more than one aircraft, you had to remember where the Trim Levers should be set for each model.
Computer radios have made setup so much easier. With Model Memories, you can set up each Model and save all the settings. Each time you fly a different Model, you just switch to the Model Memory for that Model.
The Spektrum DX5e only has one model memory, so you would need to make manual changes for each aircraft you fly. Since you have 5 channels, you can have Aileron, Elevator, Rudder, Throttle, and Retracts. You have a range of Spektrum Rx's to choose from and if you stay with JR/Spektrum, you can use these Rx's with any future radios you purchase.
The Hobby King radio you mentioned is inexpensive. It is not compatible with JR/Spektrum. It is listed as Mode 1, so the Throttle is on the Right Stick, not the Left Stick as in the more common Mode 2. I have no idea of the quality of the Hobby King radios or how many other Rx's they offer. I know nothing about their Warranty or Service. It is a 6ch radio. I do not know about the Range limits. For $32, I suppose you could give it a try.
If you want to jump to other brands, the best known ones are Futaba, JR/Spektrum, Airtronics, Hitec.
Another option would be to pick up a used, higher end, 72Mhz radio that has the option of using RF Modules. Many people are selling their older Hitec, JR, Futaba, and Airtronics 72Mhz radios to buy new 2.4 Radios. If you buy a radio like the Futaba 9CAP or the JR XP9303, you can buy a Spektrum 2.4 RF Module and it will be compatible with your AR500 Rx and any other Spektrum Rx. Futaba, Hitec, XPS, and others, make 2.4 RF Modules, but they are only compatible with their own Rx's.