Cost will be your overiding dictator. Plating in excess of .005 inch on bearing or gear seats will be a problem. Seal surfaces should be ok up to .010 - .015 plating. Welding is fine for keyways, seats, seals, but again costs will determine. Speeds will require consideration, as shafts typically will require some alignment following a weldup. Sleeving is great for seal surfaces, but need a snug fit on bearing, gear seats. The higher the cost of replacement, the more attractive becomes repair. A good repair can be considered new.