First of all, if you plan to use a good quality, known brand of motor oil, there are all the additives you need already in it. Besides, if you have a coolant leak, motor oil isn't the solution.

You wonder if it's a coolant leak. Did the cooling system not have antifreeze in it? If it only contained water, then you may be correct about a coolant leak. If it did contain antifreeze, then the clear water was evidence of condensation. A second contributor to this conclusion is based on understanding the chemistry of motor oil. Assuming it had a good quality oil in it originally, the detergent dispersants would have kept the water in suspension thus rendering the entirety of the oil drained to a "milky" looking mess had the engine been operating with a leak.

The new oil you are putting in will act as a "flush" to some degree, as the new detergent/dispersants will "pick up" any clinging moisture, and you're right to expect to have to dump it shortly...................if the engine works at all. If you had that much condensation, there's a good chance that the engine is rusted inside and may not turn over at all. Or it may turn over, but "grind" itself into a near death experience. If you want to save yourself some grief and expense, it may be worthwhile to do some tear down and inspection to see if it is all corroded inside.