Dear ccd,
It is wonderful you are considering helping your mother-in-law. If you donate now, and some years in the future she needs a second transplant, your wife might be able to donate then.
The chance for adverse outcomes from surgery (short term) is well known, and if you enter into the donor testing process you will learn all about it. Death is very very rare, but has occurred. The same is true of any surgery, or any other activity for that matter.
Fatigue post-surgery is common. I certainly experienced it. Depending on the patient, this passes in days or weeks. You will likely miss several weeks of work while you are recovering, especially if you have an active job as a coach. Scheduling surgery around a break in your work might be helpful. If you read some of the prior posts here you will gather lots of information and advice.
The change in the recipient is much more dramatic than the change in the donor. It is amazing to witness the improvement that can occur in less than one day. You might find that contributes a lot to an upbeat attitude towards life.
Love, elephant