I have looked into this and want to do it for the same reasons you listed. Here is what I have learned so far.
There are 3 options
- Electro-Hydraulic Steering Pump - Replace hydraulic steering pump with one driven by electric motor. Easiest option but still has hydraulic fluid and pump. There may be issues with the pump keeping up during quick transitions (ie drifting and autocross)
- Electric Steering Column - Replaces steering rack with one with a motor. Becoming common to find in junkyards, gets rid of hydraulics, need to convert/modify rack to manual, takes up a lot of room under dash
- Electric Steering Rack - Replaces steering rack with one with a motor. most difficult but best solution. Gets rid of hydraulics, puts weight down low. But can screw up steering geometry if done wrong
All of the above options may need or will benefit from some type of controller to add/remove assist. Usually using speed and/or steering wheel angle.
For option 1 the Volvo pump is very popular, but I hear the TRW pump in Astra is the best (not available in USA).
For option 2 I picked up a steering column from a Kia that I was going to attempt to adapt for my SN95. The column was a very simple steel tube, which could easily have brackets welded to. However the motor couldn't be clocked and I didn't think I would have the room. I never did attempt to install (also housing cracked during disassembly).
For option 3 these definitely need some inputs (usually CAN) to work. But people have made the S550 and RX8 ones work standalone. The main problem for us is the size of them. Not sure what engine you have but with the SBF the oil pump is in the front of the oil pan. The steering rack goes under the engine but behind this oil pump. So not much room for anything larger which these all are larger. With that said I have picked up one from an RX8 as it looked to be the smallest that I want to try to fit in. It also seemed to be similar width but haven't confirmed that yet. I do know it's still too large to fit in the stock location without doing some modifications. So might be a bust and waste of money.
On Facebook check out "The EPAS Swap Group" although mostly has info for S550 rack swaps, but it's growing. Lastly, listen to EP 54 of Hypercritical by Professional Awesome Racing podcast. They plan to do more podcast on the subject as well, they said.
Hopefully, this helps and we can figure out a good solution for the SN95.