take a 12 volt booster pack (one of the small pocket size ones) with you and you should be able to hook it up to the truck to move the seats, and at the same time see if the power seat functions all work. but if you don't have power seats in your truck right now, it is of no point to get them as you will have to wire it all in anyways. That being said, if the seats themselves are good, still take the power pack to move them around so you can remove them, then remove the power seat rails and get the seat only and transfer it to your manual rails.
and as for dirt, if there is no mold or mildew on the seats, I would not worry too much about it. get them home and clean them up or take them to a detailer and have them shampoo them. 3-4 weeks isn't too bad to be in the elements, a little rainwater won't hurt them that much especially if they are able to dry out.
AJ