There are a lot of great options out there. What you need to decide for both suspension and seat is what will work best for you. Ask yourself these questions:
-How will you use the car?-How many more mods do you want to make?-Daily driven?-Budget?
If you are building an AutoX/track car, you will need a seat that has more bolstering, is FIA/NASA/SCCA legal, you may or may not need new rails, you may or may not need harnesses. Non adjustible seats can be uncomfortable for the daily drive, and getting your kidneys squished on the way to a movie can suck.
Coilovers: The black art of suspension tuning can be dauting! Make sure that you are getting something that you will really use. My car is my daily driver that sees occaisonal track use, so full blown colivers would be a waste for me. I chose Nismo S-Tune coils and springs as they are a matched set that are desigened by Nissan engineers for high levels of control and grip, while providing a modicum of comfort. They are non adjustible, but you DO NOT need adjustibility when everything is adjusted perfectly from the get go. If your car is a drift only machine or a full on track slut, go for the myriad of QUALITY aftermarket setups from Tein, Cusco, Aragosta, Ohlins, KW, KT, The Progress Group, and many others. If you decide to get a full on set of coilovers, DO YOUR HOMEWORK FIRST. Read up on
http://www.sportcompactcarweb.com on Karl Funke (the new President of NASAs SE-R CUP FYI...where many the Nissan runs) on suspension tuning. Don't just get what is cheap. A quality suspension setting can really wake up your car, while a poor setup will ruin all that you love about your S13!
PS: I spent literally 6-8 months of dedicated research (NO LIE) before I made my choice of coilover. And as for seat, since my car is a daily, I use a S15 SpecR seat. Holds better than a S14, and less pricey than a GT-R seat. If you want the ultimate in grip and comfort, I HIGHLY reccomend a GT-R seat. I have sat in one, and my GOD it was amazing!!!!