from my humble experience please remember that the cushions MUST be on a breathable base, NOT ply. the moisture sinks to the underside of the cushions and turns into mould, rots the cushion covers ,and then gets into the wood.
all the built in sofas iv'e seen are too hard, proper sofas have webbing supporting the cushions, nice and comfy, and don't give you DVT of the arse.
Have you considered getting a sofa in the sales, and modifying it to fit, a bit of woodwork to the base will get you plenty of space for draws, or wicker baskets which are a nice cheat out of making draws. Presuming you can get it through the doors.
I did that to an IKEA sofa, it worked out well, loose furniture is the way forward in my opinion, but i did find out why IKEA was so cheap, it was made of card board inside, and put together by a blind brain damaged chimp in China with an air stapler gun.