Well am sure you've already considered the issue of host surrogacy (where you are literally only "carrying" a baby that belongs to the woman and her DH because it came about as a result of IVF with her egg and his sperm - as expensive, time consuming and emotionally exhausting as any IVF experience with no good stats of a positive outcome.. but potentially "easier" on you..)
Or true surrogacy when you imseminate yourself with the DH's sperm, get pregnanT the usual, cheap and cheerful way, but the baby would be biologically yours.
And actually, either way, your body and hormones would tell you the baby was yours.. and the potential stress of that..
And the legal factor which would not recognise anyone but YOU as the biological mother.. even if, actually, (re host surrogacy) you're not! (Which is very mad and outdated).
No personal experience though, just thoughts I have had re possibility of helping out a friend in the past.
Expect you've thought about all this so will bugger off and stop telling my grandmother to suck eggs as it were!
Good luck if you go ahead. I think it's a wonderful selfless gift to give someone. xx