I think the gx7 will do fine. It does have in-body image stabilization so you will have that feature on any Panasonic or Olympus lenses you choose to use. I know at least nauticam offers a port for the lens you have and suspect you can get something from other makes as well. The 14-42 isn't quite wide enough for underwater wide angle, nor is it a macro, so it isn't ideal but it is OK to start with. If you want to add a lens and do so economically, you want to look at something like a fixed 12mm or the 12-50 or 12-32 zoom or the the 45 or 60 macro lenses, depending on your interests. If you could afford them, the 8mm fisheye, 9-18 zoom and 7-14 zooms are very popular underwater in micro four thirds format. But not only are these lenses more expensive, they usually require expensive dome ports
Underwater photography is not a budget activity. In addition to the housing and port, you will want at least one strobe and then of course the associated clamps, arms, cable(s) etc.
As to housing and port brands, I am not a big fan of Ikelite so don't follow their products and don't know what they have currently. Be sure you get something that can be obtained and/or serviced where you live. With that said, I would suggest looking at Nauticam housings and ports. They are relatively reasonably priced and popular enough that used housings and ports come up for sale fairly regularly. Nauticam also offers a WWL-1 attachment that will turn a standard zoom lens into a very wide lens, but it is a very expensive accessory and I don't think it will work with the lens you have. You would need to get a different zoom, I think.