The first MGL has all you need for Airspeed, Altitude, Vertical Speed. No navigation. You would need to add a compass.
Basic navigation is generally taught by pilotage and dead reckoning. You will look at a sectional chart and draw a line from your starting point to your destination. You will then use a tool called a plotter that you will be able to calculate your heading and distance information. You pick visual reference points along your route to look for to make sure that you are on track. You will know when you should see a fix because you know your speed and the time it should take you to get from point to point. Roads can be used as check points. You can follow a road but that is not necessary even without a more fancy navigation system. Many times if you were to follow a road, you would be going many miles out of your way.
Check out these videos for a basic intro.
https://www.youtube.com/watch?v=40Ac2YD2LNs https://www.youtube.com/watch?v=cglvLVab4mM I'm sure there are probably some more detailed youtube videos if you do a search.
You can always add a handheld gps or aviation smart phone app for supplemental navigation.
The Xtreme is the first option with GPS Nav. The Xtreme is what you need plus some nice to have features. It does not have a moving map but will allow you to enter waypoints and navigate kind of like VOR navigation. It adds some more capability such as a gps derived artificial horizon or an actual attitude indicator with the purchase of the optional SP-7. You can also add the SP-6 magnetic compass sensor to the Xtreme.
The others are fancy nice to have built in moving map etc.
I left out a lot of detail about the instruments, but my main point that I wanted to get across is that you only need a compass and a chart for cross country navigation.
My recommendation would be for you to take some
flight lessons. Doing so will give you much better insight and understanding for what you need, and what you don't necessarily need but decide that you must have in your airplane.