No opinon on seats, other than sit in them before you buy them.
For harnesses, I've been running the CROWE units for the last 7? years in mine, and while they work, they don't stay tight and give me a real sense of security. (I've rolled plenty, and been protected every time, but I didn't know there was a better alternative until recently)
After sitting in Cal's race jeep at JV last November, I was sold on the Hooker harnesses that he has, with the ratchets built into the lap belt and the spring loaded cams that keep the shoulder portion tight... until I saw they were $500 per seat or $1000/ car.
PRP makes a copy for $275/seat or $550/car that copy the hookers.
http://www.prpseats.com/product/ratcheting-harness/
If even that is too rich, their line of 'standard' harnesses with spring loaded cams on the lap and shoulder belts hold the 'tightened' location that you set them to a lot better than the CROWE / RCI / SIMPSON drag race stuff that is all over ebay and summit racing, and I'd say is well worth the slight price premium.
http://www.prpseats.com/product/5-3-harness/
I re-did the interior of my buggy hoping to have room for the ratcheting style harnesses, but don't. On my list in the short term, though is the harnesses I recommended.
No mater what brand you choose, pick all 5 points. The anti-submarine 5th belt holds the lap belt on the hip bones and prevents it from riding up onto the squishy abdomen body pieces that don't take compression loading nearly as well.