"I wish I would have just gone ahead and bought one finished looking back, but damn, I can tear a Z apart with my eyes closed now. "

Boy do we know that feeling. We just finished completely re-building ours about a year ago.

As for your questions, you can use the stock seat mounts if you can adapt whatever seat you are mounting to them. That said we welded in two pieces of 1" square-stock between the sill and the trans tunnel to mount our seat to. It's a bit lower than we would have been with the stock mount, and we feel it's safer. If you fabricate bottom mounts for your seat it's important to note that you cannot both weld the mounts to the car, and also weld them to the cage because of the limits on the number of cage attachment points (8). We just have a Kirkey seat, and we are fine with it. There are a ton of nicer seats out there, but this was a place where we had limit our budget to be able to do our car.
As for the passenger seat check with the club you run with as for what they require. We started with auto-x and HPDE's with our car as well, and we used a stock seat with a 5-pt for the instructor. That said if you have a race seat, technically your instructor is supposed to have a race seat by SCCA HPDE rules (last time I checked) which dictate that while stock seats and belts are ok, if the driver has upgraded to a harness or a race seat the passenger has to have the same level of equipment. Driving with the stock seat would be a slippery experience, but you could try going with two stock seats with race harnesses if you don't want to spring for two race seats to get you through your HPDE's.

As for the diff, the LSD is the way to go, but there really aren't any easy buttons out there on the r-180's. What units are out there for sale aren't cheap. $1200k and up. We ran with an open diff for a couple of years, and it's 100% fine to get started. The most important thing is that you get seat time. With or without an LSD it will probably take you a couple of years to get up to speed, and the guys that always run up front are always on fresh ($) rubber. As for welding a diff, it depends on what tracks you run, and your driving style. The car is going to struggle to turn in tight corners if you weld it up, and to get around this you have to really throw it around, but for years many z drivers ran with them. Most guys have LSD's now, but since we only have one LSD, we plan on welding another diff up with a taller ratio for Daytona.

Congrats on the build, and good luck finishing it. They are an absolutely joy to drive on the track. Very well behaved, and enough power to put a smile on your face. Post some pics up if you get the chance. I'll try to get some pics of our simple seat mount when I get home, and add them to the thread. The forum here is a great resource. A ton of z-pilots here with a lot more knowledge than I.