the doors you have seen, are they panelled? and are they solid, or are they veneered on an inferior core? Can you see if they are made with mortice and tenon joints?
if they are solid, panelled, and M&T, they will be fine inside. They will be stronger and heavier than most doors so will need three 4-inch hinges, and the door lining ("frame") will need to be solid enough, and well-fixed to the wall, to take the weight. That sort of door has been fitted in expensive houses for hundreds of years. If they are good panelled doors, they will not warp, but if they do (e.g. in a damp old house) there is a way to bend them back to shape. It is quite easy, I have done it myself when shown how, and I am no joiner. You can't easily do that with a flush or moulded door.
You should leave them indoors for a few weeks to acclimatise to the dry conditions before painting them, or the paint will crack round the panels as they dry and shrink. The wood itself will not crack. As they will be heavy, and need to be well-fitted, it is worth asking around for a skilled local joiner to put them in for you. He will be much quicker and do a better job. Include any locks and handles that you want. Choose good hinges. With heavy doors, it is an advantage to have lift-off hinges as this makes it easier to take them off when painting, or moving large furniture. It is quite difficult to lift and re-hang a heavy door and get it back in the perfect place otherwise.