The 93 Cobra is one of the most beloved versions of the Fox body Mustangs. Its a really nicely made car that has gained some collector status. I still see mint 93 Cobra's going for $14-$16K
How your buddy says that the 87-93 cars were poorly built is beyond me. Obviously the newer cars are built on a better platform with more rigidity in the chassis and better suspension, but the reality of it is that the fox body cars were really well built for their time. I owned a 92 GT that i put 170,000 miles on it. The only reason i sold it was because the body was starting to rust, i wanted something "new" and didn't want to spend any money on the car. At 170K the engine was still running strong, and the transmission had started slipping (automatic) so i decided to get rid of it.
As for what is the best year or model to buy for one of those cars, well that depends on what you plan on doing with it.
-If you are looking for a car that retains it's value pretty well, but also costs relatively alot to begin with the 93 Cobra is a great buy.
-If you are looking for a car to highly modify with a power adder than i'd probably suggest getting something between 1989-1992. Fuel injected Mustangs prior to 1989 had a thing called speed density that doesn't lend itself to large modifications too well. (although it can be swapped out for mass air which 1989-up cars have)
EDIT: oh and as a side note, we're in the middle of doing a section for the site that is dedicated to Mustang Cobras. Specs, production numbers, pictures, etc.... Stay tuned.