First off, OBD-I ECUs can pull codes, either via the blinking LED or through Consult.

The easiest way to do this is with a S13 dual cam harness, ECU, PS pump, PS lines, and custom AC arrangement. With this, you can keep your 95 intake manifold. People like me swapped the lower intake manifold, simply because it was easier to delete the emmisions rats nest.

If for some reason you absolutely hate having free time on weekends, I guess it is technically possible to run an S14 harness and ECU. Be warned that the interconnections between the motor and chassis (F1, F9, F10) are no where near lining up. This is further evidenced by the fact that a conversion harness a S13 SR (which, layout wise is very close to S13 DE) into an S14 costs 3 times as much as normal. Now that that's out of the way, you could use your stock climate controller, and I'm pretty sure your other interior functions would remain the same. IIRC, only the wiper controls are built into the S14 harness, so you'd just have 2 sets (and you'd ignore the S14 ones). I think you might also have to add a rear O2 sensor to have a complete OBDII system, too. And yes, you'd need a DOHC cluster.

*edit* oh yeah, the S14 KA has an internal coil distributer, where the S13 KA is external. If running the S14 harness, you'll use the S14 distributer. If converting to the S13 harness, you can either source the S13 dist, or have someone like Taylor wire in the S14 unit for you.