Don't worry about 7.1 until you get a properly matched 5.1 setup first. Given that you already have a pair of speakers (albeit nonmatching) that you can use as surrounds (use the Energy speakers), I would go with the subwoofer. Even if those Bostons put out decent bass, that extra lower octave that a subwoofer provides gives an entirely new dimension to your listening once you set the thing up properly.

The Boston VR-M series is a huge step in sound quality compared to the low end Energys and Polks you've referenced. Since you're already comfortable buying used gear, you should look into buying another pair of VR-M bookshelf sheakers and try to find the matching center speaker. That will give you a matching set. The rest of the challenge is setting everything up properly (the alignment, delay, subwoofer placement, etc.).

More than anything else, your room will dictate whether or not you can/should go with a 7.1 setup. The two back surround speakers should be installed ONLY if you have a fair amount of space behind the listening position. If your sofa is pushed up against the back wall, forget about 7.1 -- you don't have the space, and improperly aligned back surround speakers will make things sound worse and lighten up your wallet in the process.