First welcome to 1200 ownership. I've raced mine since 89 or so and it has been in a circle of friends since 85. I've raced or owned a variety of Datsun's B210,510, 240Z and I like the 1200 best.
For a cheapy suspension upgrade you can use 7/8 front bar , 5/8 rear bar, lowering blocks in the back and 240Z rear spring with one coil cut out. I used this set-up on the car at SCCA Autocross and Club races from 89-92.
Since 92 I've had 280ZX coil overs , control arms and radius rods.(crossmember is modified) The rear uses NISMO springs with Koni shocks.
As for 15" rims , well , remember the 1200 came with 155/12" tires so without some flares I'd say stick with some 13 x 6 Rims. The A12 motor only puts out 68 HP so those big tirels will act like four giant flywheels and slow the car coming off corners. Eevn on a street car it is noticable. Unless you plan on putting a twin cam motor go the old school route.
For a starter go with cheapy suspension set up some 185/60 13 grippy tires(Kumho/Falken) and put on a weber 32/36 carb with a header and 2" exhaust. Also get some good brake pads. This will make the car loads of fun to drive.

Tom Grossmann