I'd definitely paint wood preserver stuff over the field shelter, it does help to keep it going longer, and you may need to re do it every few years.

Do you have any hard standing? If not then rubber matting comes in handy as a substitute, so you have a firm surface for farriers visits, and for cleaning feet etc.

Also having matting in the shelter will help keep the floor dry and even, it's up to you if you want to put some bedding down as well.

Ours have access to their stables 24/7, and mostly they use them during the summer to get away from flies, very rarely do they go in in the winter, even during really horrible weather.