I have not done this but it sounds wonderful. It’s great to hear you have both already discussed it. From your other posts she sounds supportive and a team player with you. Very lucky situation to have.
I have been researching strap on harnesses enough to know there is a wide variety. They range from very strappy and minimal, to more like full coverage briefs. There are so many different materials, types of padding, and places to adjust.
I recommend you choose the harness you want to wear, and she should chooses the harness she wants to wear. This way you each get the style and features that are most important to each of you. Comfort while wearing a harness is very important. Also, achieving a snug fit is critical for giving the wearer good control of the appendage.
Then, for your first dildo purchase, you choose the item you want to penetrate you, and she should choose the item she wants to penetrate her. This gives each of you the control over your physical situation as much as possible in the beginning stages. Later, after you have both learned a few things, it’s easy to buy a new dildo that has different physical characteristics.
I hope you both go through with this experiment, and are able to have fun while exploring how to make it work. First time may be awkward, but keep talking, and the second time will get better. Can’t wait to read more updates from you in the near future. Cheers.