HID kits are all more or less the same, the sort of cheap Japanese stuff you get on ebay should work just fine and they come with good garantees usually like 12 months so can't complain.
as for fitting, I've done it twice / three times now from memory. its one of those must do things to me, the difference, ironically, is just night and day once you've done it.
if its anything like the 156s (as I not done a GT) then you need to make sure that the bulb will fit behind the glare / dazzel cap things in the headlights. HID H7 bulbs seem to be a bit longer than standard H7 filament lamps.
H7R HIDs are a bit shorter and a lot of people have managed to get them to fit in however I have had a set of H7Rs fail to fit on a GTA. H7C is the way to go.
once you are as far as a pair of bulbs that fit, its pretty straight forward to be honest, they install like a normal bulb and you can plug the ballast packs into the standard bulb feed, two wires and it will work.
it is recommended, to help the ballast and to protect your wiring etc due to the initial current draw of a ballast on ignition that you wiring them into a harness - something I would highly recommend and always do myself.
all you need is a couple of automotive relays for about 4 quid from maplins and use the standard headligh feed to trigger the relay. you can use the relay to feed power straight from the battery to the ballast then, dead easy to do and much better for the HIDs. Just be sure to put a fuse in line on the batter feed as close to the battery as you can get it.
as for the blackline bit, when it comes to GTs am afraid you lost me there.