Cheap depends on what is available for you.
Some people use wood pallets: one for each side, plywood on the top. You might need to cover the pallets with metal hardware cloth to keep out some kinds of predators (like weasels, or raccoons that can reach through small holes to grab chickens.)
You could start with a big dining room table (roof & legs), then put hardware cloth around the sides (predator protection and ventilation). If it is next to a solid fence or a building, you might not need solid sides on the coop.
You could start with a plastic playhouse.
The hoop coops people have been discussing start with a wire fence panel. I think you've said those are not easy to get in your area, but you might look at how people build those coops and think about what you do have.
For the roost inside the coop, it's common to put a board across from one side to the other for the chickens to roost on. It can attach at each end, but have it at least a foot away from the sides and the roof, so there's enough space for the chickens' heads and tails. More space above the roost if often better, because then you can have ventilation above the roost without the air blowing directly on the sleeping chickens.
For nestboxes, you will only need one or two because the hens take turns laying eggs. The hens aren't very picky, as long as it's a good size (about a foot each way). (Exception: if your hens go broody, each broody hen will want a nestbox of her own. That only lasts while they are broody.)
Nestboxes can just be boxes (cardboard, wood, plastic) sitting on the floor of the coop. This is really simple, but makes less floor space for the chickens to walk in, which affects how many chickens you can keep in the coop.
Or nestboxes they can be mounted on a wall of the coop or supported on something.
I've seen photos of plastic buckets inserted between the rungs of a ladder
Mounting the nestboxes above the floor means you can keep more chickens in the same coop. Some coops have nestboxes that stick out and you lift a hinged roof to collect the eggs. That style is more difficult to build, so probably not what you want at present.
The chickens need food and water available all the time they are awake, and the food needs to be protected from rain. Some people put food & water in the coop. Others put food & water in the run and have at least part of the run covered to keep it dry. Some people have raised coop, and put the food underneath. I think I've seen pictures of an umbrella over the food in a run that was otherwise uncovered.
Many people add a pop door (small door for chickens to go through.) But if you don't want to build one, chickens can just go in and out through the same door you use when tending them--leave it open, and maybe tie it or put a rock to hold it open, so the wind cannot blow it closed.
The main con I can see is that I don't know much about attaching things to plastic
I know some people do it, I just don't know how.
Things needed when converting:
--cover all windows and other openings with hardware cloth (metal, holes no larger than 1/2 inch) to keep out predators while allowing ventilation.
--add roosts (maybe sit a chair or sawhorse inside for the chickens to roost on, depending on what you have available.)
--add nestboxes (could be boxes on the floor.)
About space per chicken: a common recommendation is 4 square feet per hen in the coop, but bantams are often fine with half that much, because of being smaller.
do not buy those cheap mass produced coops---Too small and flimsy as well as over-priced.
