185CM/100KG or 6'1"/220LB depending on your accent.
-Snip-
Other comments are that with the Super, you should select a board
~10CMs longer than you usually use.

My first board was a Supermodel 168. IMHO the largest person who should
ride that board is about 190 pounds, any heavier than that, and the rider's
weight will simpoly overpower the soft flex, and the board will be too
unstable at any kind of speed. The soft flex is good for learning on, and
if you were lighter I would in fact recommend it but IMHO you are just too
big for the Super 168. I am one of the people who has commented on Burtons
softening quickly, and I suspect that's because the two that I've owned
(Supermodel 168, Custom 160) really weren't designed for someone my weight
(over time, anywhere from 175 - 200 pounds). The main reason one needs to
size up is that the Supermodels had a little exatr length in the nose and
tail, meaning the effective edge would be more akin to something shorter.
Also, they had sidecuts more typical of a shorter board.

You might consider looking for the 173 instead of the 168. That might
support your weight better.... but first, what size are your boots?
Supermodels were pretty narrow. If you're a size 12, you'll need a wider
model.

1. Is this likely to be a board I can use for a while without
outgrowing it?

If you get it long enough, and value a cushy ride over high speed stability,
sure. If you want high speed stability, you'd need the 181 which in turn
will be too much board for you as a low-intermediate.

2. Is the 'over-flexibility' and degradation of these boards an
issue?

At your weight, probably yes. It was for me averaging out at 190 pounds.

3. Would I be better off with some other design in a ~160ish length?

IMHO most ~160ish boards will also be just a tad too small and soft for 220
pounds. For the Supermodel, look to the 173; if you are interested in other
boards, look around at various manufacturers' sites and see what's
recommended for 220 pounds and you'll get a pretty good idea. For the most
part I think you'll see that the boards are 163 and higher in length.
Stiffer boards will work for you in shorter lengths where softer board
you'll still be looking ion the higher 160s. Examples - in Burton, you're
looking at something like a Custom 166 or an Omen 170. (The Omen is a
descendant of the Supermodel, only a bit stiffer from what I hear and with a
little less nose and tail). You could probably go shorter in Salomon, which
tend to be stiffer (maybe a Forecast in the 163 ish range) or you could look
at one of the smaller manufactuers like Donek or Prior which tend to be very
sold rides, and maybe even go a little shorter. (I ride the Donek Wide 161,
and feel like it could support up to about 210 pounds pretty easily, maybe
even 220)

4. How would I tell the manufacture year of this board? Did the
graphics change much?

I *think* the serial number includes the model year. I can check my wife's
old Burtons later and get back to you on how to use it. Otherwise, yes the
graphics did change... I can identify the years w/ a picture but I don't
remember the designs well enough to give a description, sorry....

I hope this helps

Mike T