Agreed, many people do it this way, including myself.
I just bought my Infinity P363 Towers for my mains. If I like them, I'm going to get 2x more P363's + PC351 Center to finish the 5.0 Setup.
After I move to a bigger place, I'm going to snag a couple Epik Legends to finish up the System.
Not only does this give you time to gauge your preference between the 5.1 and 2.1, but it's also safer if you can't hear the speakers before you buy, which is my situation.
I would definitely try to give the Polk's a listen if you can. They were a bit too "bright" for my taste. I owned 2x Monitor 70's, 2x Monitor 40's, and a CS2 Center. Best Buy carries the TSi Series which is their replacement so I'm sure you can at least get a listen at them. They're not IDENTICAL but they're extremely close.
On the Sub, it really depends on your budget, imo. For a lower-priced Sub, I would go with Epik, Elemental Designs, or Emotiva. HSU, SVS, Rythmik, etc. are all great companies but you have to pay a premium to get their better subs where as Epik, eD, and Emotiva have great selections in the budget range.
Good Luck and Hope this Helps!