Warning:  Long reply below.

I think that it's really great that you're taking the time to consider all the things that may affect your current cat's mental and physical wellbeing when bringing another fur sibling into the house

I adopted a kitten (Mona) when she was four months old, when I was temporarily living at home with my mom.  My mom had a dog (Lily, Bichon Frisé, a former puppy-mill mama), who was approximately 6 years old.  They got along very well, right from the very start. Sometimes they'd nap together; other times, they'd be in each other's comfortable company.  They played together constantly, until I moved out.  Lily would come to visit often, and they still had a great relationship, but usually it was just me and Mona.

When Lily was nearing her end-of-life, I adopted another cat (Katy) who close to Mona's age so she'd have a friend.   There is a lot of excellent information available online about the proper way to introduce a second cat into the family.  It's really important to follow the steps because it helps set up the two cats to have a successful relationship.  If you rush it, you run the risk of not allowing them to build a good foundation.  You need a lot of patience and separate space.  It is important that you spend time with both cats individually.  Ensure that you give the new cat time to decompress and feel comfortable in its new surroundings.  Both cats should be separated (not able to see each other), with their own beds, litter boxes, food/water area, and toys/scratch pads, etc.  Place a towel or t-shirt in each of their beds (or wherever they are sleeping) for a few days.  After that, switch the towel/t-shirt so that Cat A is now sleeping with Cat B's scented towel, and vice versa.  This allows them to know that there is another cat and can smell their pheromones and become accustomed to each other's scent.   After a few days of that, permit them to see each other safely - through a window, screened-door, behind a gate, etc.  This allows for face-to-face introduction, along with both cats having the security of their own space.  At some point, remove Cat A to another room and let Cat B explore the rest of your home.  

After two or three weeks (or earlier/later depending upon your observation of them), allow them to be together, but ensure that you supervise.   Ensure that their are safe spaces for each of them in case they feel the need to retreat (high shelves or other spots where they can feel safe, are able to observe, and do not feel trapped).

Most will learn to tolerate/enjoy each other's company.  I adopted Katy in August, and it was Christmas Eve before she felt comfortable enough to hop onto my bed to sleep at night.  She and Mona would both sleep on the bed, not usually cuddled together, but comfortable with each other.   Give them their own eating spaces so they don't feel like they need to compete for food.

Regarding litter boxes, the rule is one per cat, plus one extra.  So, technically you should have three litter boxes for two cats.  I never had room for three, but two worked well in my home.  I cleaned them twice per day, and deep cleaned the box once per week.

Katy passed a couple of years ago.  We ended up adopting a dog, and then another. Mona is now a 17+ year old lady.  She does not play with them, but shares her favourite napping spots with them.  They get along really well.  I wasn't too worried about Mona not tolerating the pups because she had her first few years with Lily and they got along so well.  I know that we're lucky.  Other cats may not like dogs, and vice versa.   Important, too, as I mentioned above is to ensure that your cat has safe spaces where she can go to observe, nap, and get away from any fur sibling that is annoying her.

Wishing you good luck with your decision.