WRYZ926

Another option if you want a custom length cable is ABR Industries. Yes they will cost a little more but they are excellent cables. You won't go wrong with cables from ABR Ind., Messi and Paoloni, Times Microwave, or DX Engineering. Another good source that I have only used for short jumpers is the Jet brand from R&L Electronics. 
You will be fine as long as you stick with LMR400 or a quality equivalent.  Avoid the cheap stuff from Amazon and eBay.
The Comet CA-712EFC is a very popular antenna that works well. It is long at around 10 ft. The Comet CA-GMRS is another good antenna if you need something shorter. There are other brands that will be suggested.

You can find power supplies on Amazon that put out the correct 13.8v DC and have a 10 amp accessory port (cigarette lighter) that will work fine for a 20 watt radio. I suggest buying a 30 amp power supply just incase you ever decide to get a 50 watt radio to use as a base station.  
Here is one example: 13.8V 30A Bench Power Supply, Analog DC Power Supply with Cigarette Plug, Ham Radio Regulated Switching Power Supply, Noise Offset Control I know its a generic brand, but that exact power supply is sold under various names and has a good reputation.
One thing to remember is that most, if not all, power supplies are marketed at their peak power output. The continuous power output is between 75 and 80 percent of that. SO a 30 amp power supply is really only good for 22-24 amps of continuous use. A 20 amp power supply is good for 14-16 amps continuous and a 15 amp power supply is good for 10-12 amps continuous.

I would love to run longer antennas but I need to be able to park in my garage and also fit into low parking garages at my doctors appointments. I might end up getting another Comet 2x4SR for when out and about.