PS is perfect for a car left outside all the time (it probably won't offer the added shine or depth of a wax, though, on a darker car like yours). Also consider picking up some HGG to boost your sealant every few washes. Awesome product.
It can be maintained, yes. What product(s) you use depends on the results you want, and others chimed in with some options. But the biggest difference in what you're doing so far will probably be the towels you use. There's certainly something to be said about using high quality MF towels, and Adam's offers lots of options to use with DS, Waterless and Rinseless, among the other products. I highly recommend you pick some up in your next order, if you haven't already, and you'll see and feel the difference. Some of my favorites are the SST, DST, and WWT.
Also note that Rinseless diluted 16:1 is the same as Waterless, so it provides a more economical approach.
Here are some articles you should read/videos to watch, if you haven't already. They may help answer some more questions:
http://www.adamsforums.com/topic/30518-dusty-car-from-driving-how-to-wipe-her-down
http://www.adamsforums.com/topic/31693-waterless-wash-vs-rinseless-wash
http://www.adamsforums.com/topic/27631-faq-should-i-use-wax-or-sealant-on-my-car/
http://www.adamsforums.com/topic/28664-ask-the-shine-doc-ep-6-sealants-vs-waxes/