The best way to flush your engine is by following the owner’s manual, which is probably something that doesn’t come with a used boat, so let’s touch on it here in Waterblogged.
As we have highlighted before, the Alpha drive raw water pump is located in the drive, and proper flushing means using the “muffs” so that the pump can draw water up through the drive. Doing it this way ensures that the pump is actually drawing water and that the passages in the drive also are being flushed. (See Water Pump Impeller: Priority One and Salt Neutralizing for Marine Engines: Royal Flush.)
I know flush kits are available that let you mount a hose connection to a bulkhead or another more convenient location like the hull side, but they can cause water to enter on the outlet side of the Alpha pump, which would probably damage it and would not flush passages in the drive. The flush kits are really for use on engines that have a raw water pump on the engine, driven by a belt. The most common are those used on boats with Bravo drives.
Probably the best flush kit you can buy is the Mercury 44357T-2, which should run you around $25 or so. That kit has suction cups with wire that passes through the inlet ports on the sides of the lower unit gear case. It’s superior to others because the wire keeps it from falling off during the flush process, which would result in impeller damage or overheating.
Using the muffs with the wire also tests the ability of your raw water pump to draw water up and into the engine. If it has a poor seal due to the damage, it won’t draw water and that’s something you can’t determine when using anything but the muffs.
If you boat in saltwater, you’ll want to run it long enough to get the salt out. Just taste the spent water to judge whether all the salt has been flushed away.