This has been covered loads matey, use the search feature to try to find some other people's input.
Mainly though, rust, rust and more rust. No matter what they look like on top. My S14a looked so clean when I bought it, and underneath, but when you start poking around you really find out that they are made of cheese. Look out on the strut tops under the bonnet, rust around the chassis rails at the front, all along the sills, under the skirts and the the edges of the floorpan that they join to. Rear subframe mounting points particularly the front bushed mounting points and around the extra 2 bolts on each side that don't have bushes on.
Rear valance and all arches are also prone to rusting. This goes for all S13,14,14a.
I don't know much about S13s, but my S14a has suffered from sticky rear calipers and has a little VVT rattle on warm start. Youtube it if you don't know what it is, but its not really detrimental unless it does it all the time.
The engines are pretty solid as long as you maintain them well; regular oil and filter changes are vital but everything else can be replaced if it has been overlooked a little.
Stock T25/T28 turbos can be run up to around 1 bar on stock management, any higher and they'll start needing additional mods.
Standard wing-mounted intercoolers are turd, and often crack and piss boost out.
Steering rack bushes are common to perish and cause vague sloppy steering.
My gearbox rattles a bit mid-way through the revs when you aren't on the throttle, dunno what that is though it may just be mine!
Hope this helps a bit, I'm sure theres more to watch out for!
