1 & 2. I'd skip getting 80mm fans, they aren't really necessary and depending on the model can get rather loud. You probably don't need fans for the bottom of the case either. The Air 240 should handle temperatures very well, so you don't need to use up every fan mount anyway. I'd recommend that you build the PC as is and then check the temperatures. If for some reason airflow is inadequate, then you can add fans as needed. If you're interested in low noise, filling the case with fans isn't a good idea unless you have a fan controller or a means of controlling the speed of all of the fans via the motherboard fan headers.
Corsair makes quiet editions of their SP and AF line of 120mm fans, and some have LEDs in a variety of colors. They should suit your needs just fine, it seems like many people are satisfied with them. Separate LED strips are an option, but would require you to run an extra power cable or two from your PSU.
3. It depends on how many fans you want to hook up. The motherboard has only three fan headers. The H100i uses one fan connector and powers the two fans for the radiator with cables from the pump unit.
Here's a nice cable that allows three fans to be plugged into each header.
4. You can always replace the H100i fans. Corsair's SP line (
static
pressure) is better for radiators than their AF line (
air
flow) and will match the rest of your PC if you get the Corsair fans with LEDs.