if you're new to painting, can skip the bottom 2 first cause it's meant for pre-shading.
The first 2 layers mean.
when you want to to panel line washing, you have to put gloss coat first so that when you panel line wash, the paint will flow smoothly on the lines.
"must be different paint from previous layer" means, assuming you paint your base layer with lacquer/acrylic paint then you'll have to use enamel paint as panel line washing because these paints won't react together(mix). Unlike lacquer and acrylic, they don't go well together hence base layer paints are either lacquer or acrylic most of the time.. enamel also can..

there's 2 type of primer/surfacer, both are the same
gray - normal use for all colors
white - this is just to brighten your colors like yellow, etc.

if you spray the gray one and spray the yellow, it might not look so sharp therefore, the white surfacer comes in

I'll just give you simple steps of painting gunpla
Pre-assemble kit -> Sanding -> primer/surfacer -> base paint -> gloss coat(can skip if your base paint is alr gloss) -> panel line -> topcoat(gloss,semi,flat)(can skip if you want) -> decals -> final topcoat of your choice

hope that helps

lol... any kit can be painted dude..
ecoplay is just re-used recycled plastic.. that's all...