Taking this unit apart is simple. Davie suggests splitting the ring nut but I've never found that necessary and since you are going to need a tool for retightening the nut, you might as well use it for undoing the original nut. If the original proves to be un-removable then you've lost nothing trying.
If you're not into the lower unit yet, for info; when you turn the drive shaft it has a small bevel gear splined driven on its end held by a nut. This gear turns both the forward and reverse gears all the time the engine is running. The inside part of both gears is formed as part of a dog clutch. The gears are not joined to the propeller shaft but the shaft passes through the centre of the gears and they are aligned by the shaft. The end of the gear change rod is a wedge but it is steps rather than an even wedge. When the gear change rod is moved a spring-loaded plunger follows it. Between the gears is a sliding section around the propshaft and splined to it. This sliding piece is moved, via a few bits, by the gearchange rod/wedge and on its ends are dogclutch teeth which can mesh with the ones in the gears. Since the gears rotate in opposite directions, then the sliding section meshes with the front gear ( the one in the nose of the lower unit) it will transmit the drive from that gear to the propshaft. Likewise from the rear gear but in reverse.
So, it's all fairly simple except there are a few bits which can fail which I've not mentioned and one of them is a crosspin in the sliding section as Davie has mentioned.
The bearings are normal and available from bearing suppliers so no problem there if you feel they are needed.
Water sealing is done straightforwardly using O-rings and a gaiter around the rod.
You appear able so as a first try I suggest you have a go at it. You may find it's going to cost very little in parts to repair.
Most of the parts can be removed without removing the drive shaft.