If everything goes right then yes, you don't need the bushings. If something is not exactly right, you will be very grateful to have the bushings in place.
The center bore size varies. These cars a lug centric so Nissan wasn't concerned with a exact, machined hub center. If you look there is a radius that will interfere with a tight fitting wheel centerbore so be careful.
http://www.asbbearingsonline.com/
Standard metric cast hard bronze bushings. Depending on flange thickness you'll either need the ZMB1012-10 or ZMB 1012-15.
15 x 8s pretty much require coil overs and for racing there is a noticeable difference in performance between a 7 and 8" wheel when running 225s. To get the most out of that wheel and tire combo you need to get 1/16" rear toe in and at least 1.5 degrees of negative camber in the rear suspension. It's all about the back of the car with 15 x 8s.
225/50-15s are a perfect performance size for a 15 x 8 wheel. I ran that size for years on mu autocross and TT 240Z. I've also run 225/50-14s on 14 x 7 wheels where the rules required a 14" wheel. Those worked great also.
Most likely a snap ring style R200 CLSD with the flange drilled for 12mm bolts. For an S30 application you would need the snap ring halfshaft style side axles and 12mm to 10mm bushings to adapt to the 10mm ring gear bolts in a S30 R200 ring and pinion.
Jason Rhoades ran the stock S14 ABS system on his SM 240SX and was very happy with it. I have not installed ABS on a S30 myself. It sounds like a good project.
The issues you are concerned with (front wheel lockup) can be dealt with in much simpler ways. Its all about balance and most rear disk brake conversions don't have enough rear bias. My BSP 240Z and Vic Sias' BSP championship winning 240Z both had outstanding stock brakes and neither of us had a wheel locking problem with 225, 245, and 265 width Kumho V700s and Hoosier A3s.
There is no real world hard number. Every 40+ year old car is different. Some cars you ll get 1" of lowering, on others the car will raise 1/2". Cars have AC, shot factory springs, 200 lbs. of stereo equipment, fiberglass hoods, etc...
And to echo mike... Spend good money on your safety equipment. A helmet, seat, harnesses, HANS, suit, shoes, and gloves are a minimum if you are going to run your car on a race track.
Shit happens.
Glad you got out of it Mike. Instructing is probably the most dangerous thing you can do on a race track, especially in the intermediate open track run groups.
The Quaife diff requires suspension compromises to work properly. When ran the diff on my ROD I had 375 front and 325 rear springs with a 1" front bar and a 5/8" rear bar. The front bar was full stuff and the rear bar was full soft. My big advantage was triple Penskes with a low speed compression adjustment. That let me stand up the outside rear to keep the insider rest loaded.
Switching to an OSG Supetlock let me focus on suspension tuning for faster corner entry and mid corner while keeping a good corner exit.
Here on the west coast we would strip that car and scrap it. If you've done rust repair on an old Escort and an old Lotus then the S30 should be a piece of cake.
I've only had one experience with one of those batteries and it was all bad. I was supporting a customer at a race and he insisted on one of those in his 350Z race car. Ultimately we figured out the thing had an internal circuit breaker which the customer tripped when he unhooked the battery charger. 15 minutes after the green flag we had finished swapping in the stock battery but that didn't matter. He threw his super cool 5lb. battery in the trash after I told him it would be a real bad thing if he lit it on fire.
On the ground:
14 7/8" front, 15" rear
Factory droop at the shock is 3.25" Measuring droop at the hub center is meaningless due to the variable hysteresis of the rubber bushings.
Look at it this way:
A 151hp (factory rated) L24 makes 62.9 hp per liter. A 350hp (factory rated) 350cu. In. SBC makes 61.4 hp per liter. The L engine is, from the factory, in a high state of tune.