The plastic will become brittle if you mistreat it. It doesn’t do well in direct sunlight, and radiation (for AFB treatment) will make it brittle too. However, Cedar and Stu (inventors) have frames which are 3 or 4 years old, and they are still working absolutely fine. So I don’t think your beekeeper has a strong point.
Only you can answer that - we don’t know him! 
However, one possibility is just to tell him that you want help managing bees in an 8-frame Langstroth hive. The Flow hive is just a standard 8-frame Langstroth with a different way of harvesting. Everything else about taking care of the bees is the same as in any traditional hive. You will need to get the bees established in a brood box before you put the super on anyway, and that can take anywhere from a few months to a year, depending on your climate. If you are getting two hives, why not get one of each - one Flow and one normal Langstroth. Surely he will support you with the standard Lang. 
I think foundationless can be very tricky for beginners. I would advise starting with foundation, and progress to foundationless once you have experience in handling the frames and bees.
It will depend on your local climate, and when the best nectar flow happens. Here in southern California, the flow is mostly over by late May, unless we get some out of season rain. In summer, new hives would struggle and probably would need to be fed. In some subtropical climates, there is good nectar flow year round, so starting in summer is fine. I would ask your local beekeeping guy what he thinks for your climate and forage availability.