First of all: I'm fairly new to UW photography, so take everyhting I said with skepticism.
Well I don't want to discurage you, but as my collegues said I'd usually not suggest taking a camera under water before you don't have to think about buoyancy anymore but it just works for you. In my experience this usually takes at least 50 - 70 dives depending on how well the person handles his / her equipment, how quick a learner a diver is and especially how often he / she is in the water per month (70 dives in 10 year is a whole other story then 70 in one year).
That said regarding the cameras you have, it HIGHLY depends on
- What you want to photograph
- What results you expect
- How intuitive you want the housing and the setup
- And above all: How much money you want tho shove into the rig
If you have a shitload of money, want to do high quality UW photography and don't care about transportability. Then I'd recommend a Nauticam NA-D750 with your Full Frame Nikon D750. Depending on whether you want to do wide angle or makro a matching port (e.g. Nikkor 105mm VR MIKRO Lense with Nauticam Poart 87 for Marko). And a set of strobes that suit you need (e.g. Sea & Sea YS-02). That setup would get you almost anywhere. BUT: It will cost you (new) in the range of 10.000€! That said there a a lot of areas where you can save money (buy used! Be patient till your equipment is available used somewhere. Keep an eye on the classifieds here). Especially if your expectations aren't to high you can start out with your Lumix lx5 and a small housing. Since you won't have to put to much money into that, you can go crazy later ;-).
In my opinon: Do not save on the housing. That will frustrate you under water quickly. It is for example possible to make beautiful picture in shallow waters without a strobe. But with a crappy housing you might miss opportunies or even lose a whole dive because some setup of the camera is incorrect and the buttons are not available on the housing to change them.
Regarding the lense, prime lenses are often easier to handle (no zooming around) and have better aperture values (which is very important under water.. always not enough light ;-)). Makro is easier for a beginner (or so I heard) and the ports are usually a lot cheaper (wide angle needs frickn epensive dome ports).
Hope this helps a bit. The pros here propably have a few more and better tips