As far as the seat is concern, your best bet it to go with something like a T11 from Tillett. That’s a great all-around seat that works well in all karts. I’m 5’7″, 29″ waist and I use a size 2 seat with some padding in it. The best idea is to sit in a few seats if you can to get an idea of size since everyone is a different shape.
I also prefer to buy a seat and add my own padding to fill in the space and soften pressure points. For that, go to Target or Walmart or something and grab some interlocking foam mat like this: http://www.displays2go.com/P-14339/Portable-Floor-10-x-10-Charcoal-Grey-Foam-Mat?utm_source=google.pla&utm_medium=cpc&utm_campaign=GooglePLA&gclid=CMLPjsjXqMMCFQwaaQodUFwAsw and chop it up to fit the seat. Works great.
Sprocket carriers all do the same thing, just get a TopKart one if you can, then you know it fits at least.