If I install a package from an deb-file, because I want a newer version than available in the repositories, will they get updated when in the ubuntu repositories more recent versions are available?
Yes it will. The package will appear in the "installed locally" section on synaptic. When a newer version update appears in the repositories for that package, it will automatically be updated and so will be moved to the "installed" section.
If for some reason you want to keep the package installed and not update (not recomended) you can "pin it" trough the synaptic menus. That means you're telling the system to stick to that version ignoring further updates. This option is in the "packages" menu item, I'm sorry I can't upload an screenshot right now, but it's easy to find.
In case you don't have synaptic installed:
And search for it in the dash.