I just did this actually: ordered a rack from rackdoctor and did the DIY. It's not terribly hard.
The hardest part is cleaning up the mess the steering fluid leak made. The only hard part of the install is going to be difficult is trying to match the steering rack position on the old one to the replacement.
If it is too far off your alignment will be so out of wack you won't be able to make it to the shop so I would pay close attention (so close I used a micrometer from work to match and I was still off) to the alignment.
Get new bolts and nuts for the rack. I did not replace the o-rings on the hard lines...didn't realize there were any, but I did replace the copper washers for the hard lines going into the rack. If you go on real oem, they do not list the part number (they are blank) for those o rings. What I did was look at the real oem listing for the e82 1M. That car had the same exact steering rack as the M3 and they matched and fit perfectly.
Where you will need o-rings is the two o-rings on the servo controller. A new servo controller does not come on the rack doctor steering rack so you will have to use your old one. The problem I had was finding the right o-rings because the size isn't available anywhere. I'm kicking myself for not documenting it now.
I bought this kit from amazon:
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
I think what was included fit one of them and then I bought an assortment of o-rings from "The O-Ring Store" and obviously only ended up using a couple. The problem is that o-rings in kits tend to be too large for this application and you don't want them to be large. The servo controller should kind of click or snap back in...then the two screws just hold it in place. You shouldn't need to screw down on the o-rings to get it in there. This is the assortment I bought:
7 x 2mm X 18mm (NBR) Buna-N 70 Duro Metric O-Ring (N2.00X018) $0.56
5 x 1.5mm X 17mm (NBR) Buna-N 90 Duro Metric O-Ring (H1.50X017) $0.50
5 x 1.5mm X 18mm (NBR) Buna-N 70 Duro Metric O-Ring (N1.50X018) $0.50
5 x 1mm CS X 18mm ID (NBR) Buna-N 70 Duro Metric O-Ring (N1.00X018) $0.55
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Sub-Total: $2.11
USPS (First-Class™ Package Service): $6.13
Total: $8.24
If you have questions let me know. I have about 3-4K miles and no leaks so I guess I did it right...