I've had several powerline kits, and have been impressed with the results versus wireless networking.
To answer your questions...
1) This is impossible to answer until you get the adapters in, to be honest. I've never had a kit fail to work outright, if your wiring is old or there's a lot of "noise" on the circuits then you should expect a performance drop though.
2) See above. You may see a loss of performance but it's impossible to say before you give them a go.
3) It's possible that the signal will go through different circuits. I never had a security concern with mine because my neighbours didn't use anything of the sort. Most do seem to come with some kind of encryption nowadays, whereas the kits I've used didn't because they're quite old now.
4) Again, it's impossible to say as it depends on your wiring and circuit layout. I only used mine for an internet connection of 30Mbps, and they handled that perfectly. I never used them for file transfers so I couldn't comment on the network performance. From other people's experience, I think you could expect close to their advertised rate (e.g. if they advertise 500mbps you should get close to that) in ideal circumstances. That may sound like a silly statement but you don't always get what's advertised, with powerline you seem to mostly get quite close though.
5) Homeplug AV seems to be more common than the others, at least around here. I would go with what's most common as you then have more options for extending the network with compatible adapters at a later date, if necessary.
6) I've been out of the powerline game for a while now, my last kit was bought about 3 years ago and the kits I had before that were even older. The impression I get is that the cheaper brands have improved vastly. I had a Devolo kit (one of the first on the market) which eventually died from overheating, a Linksys PLE200 kit which also died from overheating, then a second PLE200 kit which is still running fine today. Devolo, ON Networks, and Netgear seem quite popular - also, it's worth looking at TP-Link, they used to be a bargain basement brand but their newer stuff is pretty decent.
