Hi,
The price for a rack and pinion unit from ZF will run $577-586, plus $100 core charge, depending on whether you have airbags or not. The remanufactured ZF units do NOT include the inner tie rods, boots, or lock plates. You would have to either re-use yours, or purchase new ones. But if there is nothing wrong with the boots or inner tie rods, there is no reason to replace them.
Whether you go with A1, ZF, or another brand, it will be a remanufactured unit. I have used A1 reman rack & pinion units many times with great success.
As long as the rack&pinion in yours right now is the original unit, you do have the option of ordering a rebuild kit:
Here is website for above parts: http://www.autohausaz.com/bmw-auto-parts/index.html?gclid=CJWHw_axq50CFSANDQodRBlXig
You would just need to find a shop that knows how to rebuild the rack and pinion.
Labor time will be around 4 hrs
Your vehicle has outer and inner tie rods, so you need to check to see which were replaced, but if he didnt mention them being bad, you should be fine using the existing ones. Like I said, I have had great success with A1 Cardone products over the years. I have not had one come back with a problem within the warranty time, so if it were mine, I would feel perfectly comfortable installing the A1 Cardone unit.
As far as how much more to invest in it before its too much, it depends mostly on how long you want to keep it. I would probable put the cut-off at either engine rebuild or transmission rebuild. Other than that, most repairs are reasonable and as long as engine and transmission hold up, worth the repairs.
Automotive Repair Shop Manager
Automotive Repair Shop Manager, Technician