Competition seat and harness rules vary quite a bit between different sanctioning organizations. Some require FIA seats & belts, some only require entry fee payment and safety is entirely up to you. The first step is to figure out who you plan to run with and what they require... then go shopping.
If you want to run DEs and keep your stock seats, consider a CG lock. I use them with students and it really helps to keep you in place while still running a stock car and street legal belts.
Lots of different opinions on FIA certification. FIA rated seats are not necessarily stronger, faster, or better. The sticker just means this seat model has been independently tested and meets FIA standards. Having seats tested is expensive and FIA seats always cost more. Ultimately it's your butt in the bucket so you gotta decide how important spending the extra $$ for a seat that has been tested is to you.
About FIA testing and regulations:
FIA Standards Regulations
Personally I wear Snell helmets, SFI shoes and clothing, and sit in an FIA seat. YMMV.