Is your horse showing any signs of disease at this time or was the bloodwork done to obtain baseline values for your horse? Low potassium values on the chemistry can be misleading, especially if the horse has received Lasix (furosemide, the diuretic used for bleeders) or a bicarbonate "milkshake" recently. Both of these things can cause decreased values. Other things that can cause low potassium values (hypokalemia) are the potassium moving into the cells from the extracellular fluid so that it appears low on bloodwork, excessive sweating, diarrhea, or anorexia. If your horse is not showing any clinical signs I would not be overly concerned. Feeding good hay
and grass and offering a salt/mineral block will help to restore potassium values; if your vet is concerned about overly low values without an obvious cause, he or she would be able to administer an appropriate IV fluid balanced to meet your horse's needs. I don't think that giving a potassium "drench" is something that you should do, as your horse likely does not need it and you would be unable to determine the proper amount of potassium to give (and could cause more problems than good). I do not know of any products formulated for this purpose, although there are probabaly companies selling this type of thing.
Low BUN (blood urea nitrogen) can be caused by hepatic insufficiency (although this is a really variable indicator in horses), anabolic steroids, low protein diets, and adminsitration of large amounts of IV fluids (or it may be low for no appreciable reason at all). Most of the time we are not concerned about low BUN values, it is the higher values that worry us as they may indicate kidney disease.
Low bilirubin in horses is also not much of a concern to us (we worry about high levels, although horses that are off feed for a day or two can also have high bilirubin levels). I cannot think of any good reasons why hypobilirubinemia would be significant.
Horses with liver disease
will show high values of GGT, CPK, SGOT(AST), bilirubin, and bile acids. They will often be sick and may show signs of photosensitization. I assume that since you didn't mention these things that they were normal on the chemistry.
The main point is that you can't look at the individual values on the bloodwork in isolation. You need to put all the parts together to see if they make sense and mean something real, and I don't think that they specifically do in your horse's case (although I would be unable to say for sure with examining your horse). If your horse seems otherwise healthy, I would not worry so much about the things you have noted here. Please let me know if I can be of further assistance.