it's very simple math. write down the list of changes and give an estimate of how long it will take and how much it will cost for each. write down current age and abilities and determine if you will live long enough to get the list accomplished. use appropriate wiggle room for each person's contribution to the TEAM EFFORT (ahem)... <-- make sure it is understood that all on the team must contribute as appropriate. where TEAM EFFORT is not possible then include much larger estimates for both time and money to get things done by contractors. see the eyes roll on the floor. pick up eyes off floor, rinse off, reinsert carefully.
have some tea and a cookie and see what else can be negotiated.

good luck and many hugs to both of you.