There are so many factors into deciding when/if to have another baby - finances is one, though it certainly should be the main factor unless money is so tight that you risk losing the basics - your home, food, utilities. Age is another, though 31, and even 35 is not too old to have a child - technically, 35 puts you in a higher risk category, but I personally think that's based on old medical standards, and doesn't take into consideration the modern day health reality. I had my child last year at 38 with no problem, and have MANY friends who had their first after age 35 with no issues. I also have several friends UNDER age 35 who had problem pregnancies and a miscarriage.
What you should definitely consider is lifestyle issues - how long do you want to be a SAHM mom, and how old do you want your youngest to be at that time? How tight of a budget are you willing to live with? How close emotionally do you want your children to be when they are young (do not expect your children to be too close if they are more than 4 years apart - there will be too much of an age gap for them to enjoy the same things when they are young).
For me, I think 3 years is a perfect age gap between siblings. You will only have one baby to buy diapers for at a time, you will be able to use your 1st baby's crib for the next baby since your 1st will be in a regular (or toddler) bed by then (same for your highchair, stroller, and other baby gear), your oldest will be able to help a bit with the baby, you'll be able to leave your oldest with a friend/babysitter if necessary without much issue, etc., and yet they'll still be relatively close in age that they can share friends, toys, etc.
Good luck with your decision!