The 12-50 is an alternative to the 14-42 for general purpose shooting. It is not an alternative to the 9-18 (more on that below). As a lens, it is not that much more costly than the 14-42 and I do recommend it highly for on land use as it is weather-sealed, has better optical quality, and a very, very nice macro function that is "true macro". It also has a power zoom option which is excellent for video, if video is important to you. Once I got mine, I sold my 14-42 and not only don't miss it, I am very glad I made the switch. Actually, although it is that much longer than the 14-42 which extends in shooting mode, it is very light weight and does not off-balance the camera, although it does not "shrink" like the 14-42 for non-shooting storage.
For UW use, it is the best all-around lens going, but only in the dedicated Nauticam Port/Gear. Nothing else even comes close. I went with this set-up and I can say from using it that it is beyond amazing with its versatility and ease of use. However, I know that cost is an issue for Storker and the Nauti port/gear is $800US.
Storker, before you say that is too much, let's compare the numbers for "Step One":
12-50 lens versus 14-42, $200 more (comparing prices of the camera/lens kits) and you have all the advantages inherent in the 12-50.
The dome port for 14-42 is $450. The 12-50 port is $450. So, equal.
Adding the dedicated macro/zoom gear for the 12-50 is $350 more if you get the gear "packaged" with the port. BUT, that replaces the diopter and macro adapter for the dome port of your 'step 4', which will total about $400 (200 for a quality diopter, 200 for the adapter). In addition, you have full macro capability at "step one"
So, you have $550 more up front, but avoid the need to spend around $400 later on, and you have instant switching between semi-wide, a versatile zoom range, and true macro for tiny things (you can fill the frame with a 24x35mm object like an SD card). So, in the end, a $150 or so extra cost. There is just no comparision in terms of the benefit you receive for this small extra expenditure (small in comparison with all of the gear).
So, if I really wanted to offer what I believe is best, Step One would be the 12-50 with dedicated Nauti port/gear, and a strobe. This will cover probably 70% of everything you will shoot, with one lens and one port that you never have to change, and no diopters to carry during the dive, screw on and off, or use with an expensive flip adapter. It would be as "seamless" an operation as you could ever imagine and, really, I can't imagine you needing anything else for quite a long time.
If, later and as you get more experience, you become more dedicated to macro and ultra/macro, the Oly 60mm macro fits in the same port.
If, later, you want to add wide/ultrawide, get the 9-18 and port and you are done.
But, with the 12-50, there will be no hurry to take these steps.
There is a post above that says the 12-50 only gives 1:5 magification. THIS IS WRONG. In macro mode, you fill the frame with an object 24x36mm. I have a full-frame-filling shot of an SD card. On a 35mm camera, this would be 1:1 magnification, equal to the very best 35mm macro lenses. This is an incredible feat and capability to have in such a useful general-purpose zoom, both above and under the water.