Hmmm, I'm a bit confused, the link doesn't seem to match the description, one thing being a 323i vs. a 320i... Anyway, I had my '82 320i for 13 years, and had a ball with it, over 300,000 miles of fun. Very few problems, aside from those normally associated with so many years, e.g. replace worn steering rack, shifter bushings, etc.
There were two areas susceptible to rust, the rear trunk panel where it meets the bumper, and also the seam toward the front of the trunk, by the back of the back seat, so I'd check those areas carefully.
As to the power, that didn't present itself as an issue to me, in part because I think standing start acceleration runs are stupid, and also because that 4-cyl. engine was a delight and could go into its own version of afterburners at about 3000 rpm and above, and let's face it, anybody who doesn't coordinate his gears with his accelerator pedal can't call himself a real man.
Then again, my car didn't have a/c (which was fine with me, the ventilation system did a fine job), so that may have made a difference.
As I alluded earlier, this car is a lot of fun in the dry, but you hafta curb your enthusiasm in the wet, and in snow, and if you do use it in the snow, be sure to add some additional weight over the rear axles, up by the front of the trunk, to gain a bit more traction. I don't know whether it's the weight distribution, the suspension, the power train, or whatever, but these things are twitchy in wet weather. Something substantial in the trunk, but not overly heavy, like a few copies of Schopenhauer or Heidegger, oughta do it.
As to pricing, I dunno, I'm not sure these cars fall into any informal designation of "classic," and the $5000 may be on the high side, but if it is in top notch shape, obviously it'll command some premium. I think they cost ~$13,000 back then, so paying 5/13 of the original value of a 24-year old car will require some careful consideration.