The thresholds for BMW E36 cost about one to two hundred dollars. Work on welding will cost you half a hundred or a hundred dollars maximum. So why strain yourself and take a job in which you do not understand? Everyone needs to do their job.
As the rapids BMW E36 are not load-bearing part of the body, if you wish, you can close your eyes for a while on the rust and do decorative stickers on the outside, even giving the car a certain status of tuning... But the problem lies in the fact that if it already reached the digestion of thresholds, it is all quite sad and not only where it is visible to the naked eye. As a rule, rotten sills mean and rotten the bottom of the car near them.
Ponyatno that to repair the bottoms and thresholds will require much more patience, skill, and time. You have to pull everything out of the cabin, even the door will interfere. Lift the carpet, loosen the accelerator pedal, stand, seat, cut metal, to cook, to paint, handle means for protection. Still easier to raise money for repair and send the machine for repair reliable repairers-welders.