I would not go for anything from a brand that is not one of the known & reputable ones. The sort of bivvy that is right for you depends on how much gear you are going to be taking as well as when you are going to be fishing (i.e. if you are going to do sessions through the winter then the bivvy would be different to one suitable to sessions through the summer).
I fish all year round and in some atrocious weather, but the shelter I use can be used regardless of conditions without any problems. I use the Nash Strongbow bivvy at the moment, and only find that the winter skin gets used when extremely low temperatures are expected in Winter. 99.9999% of the time, the door is never closed up so no problems with condensation, and not noticed any leaks. More than enough room for a weeks worth of kit without being overly massive so it doesn’t fit into relatively tight swims. You should easily be able to pick one up with a winterskin for under £200 (and have change!). Also got good headroom for when you are sat on your bedchair & have superb view of the water with the door rolled down instead of up (as most others do). Can quickly be erected with no pole threading required, and when pegged down is solid as anything else out there. One of the other important things is that the bag is plenty big enough to fit the bivvy back into with room to spare!