I had a BOSS GT and got quite a bit of RF interference, especially on high-gain presets so I went to individual pedals ... for a while. But I decided to get the HD500 because I've always like Line 6. Here are my answers to your questions:
I don't know about your Boss Looper, but if it is line-level, why not? However, the HD500 has a looper of its own. It may be worth a look.
I connect my HD500 to my computer all the time and it's worth its weight in gold to me. I hate dorking around with the little knobs and buttons that control a zillion settings when a computer interface will do the job much more quickly and intuitively. Once I get my presets dialed in, I can live with a little dorking around with knobs and dials :)
There are tons of community presets on CustomTone. You should consider these starting points because they are both someone else's idea of what sounds good, and may be colored by your rig.
There are L/R outputs for driving up to two amps, two XLR outputs, MIDI I/O and yes, there is a headphone jack (1/4").
I love the HD500. I've used it in my band and the only problem I had was the same one I always have -- when I plug into someone else's gear, I need to neutralize the effects they have on their amps. But really, to appreciate the HD500, you have to try it. Not for a little while, put some time into it. GC might give you some time to goof with one if they have stock. It's worth it to know whether you "get" their tone generation model.