Honesty, the best thing to do is focus on condition and maintenance records first. Because Porsche offered a bunch of options throughout turbo production, it’s possible for any one year car to have a lot of differences to another car of the same year. Some of the more desirable options that span the early years (86-88) included: lsd, trans cooler, sports seats, sports suspension, sunroof delete. As for standard or “stock” differences between the years, here’s what I can think of:

86 v. 87 turbos: Wheels (offset,f,r), struts (f), strut top mounts (f), spindles (f), control arms (f), steering arms (f), hubs (f,r), calipers (f), and brake rotors (f,r).

87 v. 88: iirc they were the same.

88 v. 88S: Turbo (247 v. 217bhp/250 ft lb v. 243 ft lb), DME, struts (f,r), torsion bars, bushings (f,r), sway bars (f,r), wheels (forged CS), transmission (stronger/cooled), lsd (40% lockup), brake rotors and calipers (928 S4), ABS (standard), sunroof delete (row).

88S v. 89: iirc they were the same (89 last year in the US)

89 v. 90/91: varied by country, some 88S standard changes were retained, some were only available as options.

Buy the best one you can afford, and know it will require money every year on into forever, whether you keep it stock or mod it.

What are your goals for ownership?