This is one of my favorite topics. I've been at this several years, and I have tried tons of different products for dunking and conditioning. If you are rehabbing old bags, the best thing you can do is get a high quality conditioner, Blackrocks, and a horsehair brush.
As far as a soap for dunking, I don't think it makes much of a difference. I'm now using Castile soap because it is gentle and doesn't make too many suds. My second favorite soap for dunking is Woolite. I like Woolite because it is also very gentile and rinses out easily. I've tried soap specially formulated for leather, and I wasn't impressed. Don't waste your money on anything special. Just about anything should be fine. Personally, I find it hard to rinse out regular dishwashing liquid like Dawn. If you don't rinse the bag really well, the leather can get stiff. If you use something like Dawn, just be sure to rinse until the water is clean and clear and no soap is left in the bag. I've also been adding a bit of vinegar to the bath when I dunk.
As far as conditioners go, for really dry bags or old bags, my favorite product is Montana Pitch Blend Leather Oil and Conditioner.
http://www.montanapitchblend.com
Leather Therapy is a close second. I think oil-based conditioners do a better job than cream conditioners in really getting into the pores of a bag, which you need to do if the bag is super dry. (Note: both of these products will darken lighter bags.) I also just love the feel of hand oiling a bag.
I usually use cream conditioners or what I would term mid-level conditioners on bags that aren't so dry or after I've oiled them and they've started to come back. Leather CPR is a favorite of this group, and I think it does a good job. I also think Cadillac is good. Two mid-level conditioners are Skidmore's and Leather Honey. I love Skidmore's. I've had mixed results using Leather Honey on
Coach bags, but it is the only thing I use for my older Balencigia. If I had to pick one, I'd say go with Leather CPR. It's tried and true. Second would be Skidmore's.
For finishing, Blackrocks is a must in my opinion. It gives bags a glow that you can't get from anything else. Obenauf's LP can also be used for finishing and waterproofing, but isn't as good as Blackrocks as far as I'm concerned. (If you are working on vachetta, however, Obenauf's is a must.)
Get a good horsehair brush. They are cheap. I find that brushing a bag makes a huge difference. It helps get the conditioner into the pores of the leather.
Finally, I highly recommend reading through all of this thread and all of the old thread. I know they are long, but the information will help you tremendously.