if you're just going to "get to that point [where you] will get a rear trailer" then just get the rear trailer and avoid all of the middle pieces. the trailer does everything the sidecar does, but as others mention, it is more versatile.
fwiw, i find most vehicles give me much wider berth when i tow my trailer than as a solo rider, making the "protetcive" advantages of the side rather moot (the sidecar is actually going to run over more roadside debris, or set you much further into traffic than you'll want to be, or both; not to mention the 'protection' your body offers to the sidecar is pretty insignificant).