Hello
I think with windows you can just fit the sides and dont even need to do the roof ? (sides are easier to transport) and I right have not done that for 20 years.
Seats, do you mean side facing or front facing. Side facing seats just bolt to the wheel arch things and cost peanuts. Front facing, like they say buy anythign and wel in or get some propper ones from an advertisier in any LR mag (Exmoor etc) although they will end up costing more than the car !!!.
Im not sure what the Law/MOT situation re DIY seats and anything to do with Kids in the back etc, maybe "if the seatbelts are securley fitted then its ok" I dont know if you can take your kids in the back on side "bench" seats these days.