I found independent shops were extremely good - tended to have a smaller number of staff but with manufacturers training on all the stock they carried, and with stock they believed in and new inside out. They also have a personal reputation to maintain.
I, too, have a recollection that the lie-flat car seats have had some dubious safety reports.
We chose the Recarro infant carrier as it felt a lot more comfortable for DC than the Maxi Cosi. It was great, except she was quite long in the back and broad in the shoulders, and didn't fit after 7 months (although we tried her in a Maxi Cosi at that point and she didn't fit in that either...) She's now 16 months, 24lbs and still rearward facing in a Concord Ultimax - fantastic now but I'm not sure I'd want to put a newborn in it.
If you do get a 0+1 seat, make sure you can actually get a baby into it when its fitted: We looked at the Britax First Class and it's HUGE - the gap left between the door frame, seatbelt and seat was tiny with the seat rear facing and I don't think DD would fit anymore.
I'm not sure IsoFix points can be fitted retrospectively - I thought they were added at point of manufacture, although all new cars now have them. My understanding is that an IsoFix fitted seat isn't automatically safer than a seatbelt seat - but its nigh on impossible to fit badly whereas its relatively easy to fit a seatbelt seat wrong and make it unsafe. As long as you can fit the seat correctly, a seatbelt seat should be just as safe.