My plant came in a 4.5 x 5-inch pot. The braided trunk is about 8 inches and the whole plant is about 18-20 inches tall. When do I replant it and is there a certain kind of soil needed?
Repotting should be done every two years.
Choose a pot that is 1 or 2 sizes larger then the existing pot.
Repot during the summer months while the plant is actively growing.
Use a commercial potting mix that is formulated for cactus and succulents.
https://www.gardeningknowhow.com/houseplants/pachira-money-tree/growing-money-tree-houseplant.htm
How big can my Pachira get? Do they do well transplanted and how tall will they get?
Indoors this plant can grow 5 to 7 feet.
https://www.gardeningknowhow.com/houseplants/pachira-money-tree/growing-money-tree-houseplant.htm
Money tree getting brown leaf edges.
Brown leaf edges on a Money Tree can be the result of either air that's too dry or insufficient light. Below is a link to a page about growing Money Trees - see the Tips section for suggestions.
https://www.leafari.com/braided-money-tree-planting-guide.html
In reference to the cuttings of a money tree, do I plant it in dirt right away or root it in water first? I have a cutting from my tree.
While cuttings will readily root in water, they normally don't transplant as well as those that have been rooted in soil. Therefore, you may want to go ahead and start your rooting in soil right from the get-go for stronger root growth.
How does the pachira money tree reproduce? Can the tree reproduce?
The easiest way to reproduce this tree is by taking clones, or cuttings. This can be done relatively easily. These two articles will help you.
https://www.gardeningknowhow.com/houseplants/pachira-money-tree/growing-money-tree-houseplant.htm
What is causing my pachira aquatica to ooze sap from the leaves? what can I do about it?
This is transpiration and may be due to overwatering.
Here are some articles with more information.
https://www.gardeningknowhow.com/houseplants/hpgen/houseplants-that-increase-humidity.htm
https://www.gardeningknowhow.com/houseplants/pilea-plants/pilea-chinese-money-plant-info.htm
Firstly - I'm not even 100% sure it's a pachira aquatica... It seems to have a really, really dry trunk (especially at the base of the plant) but still happily growing new stems and leaves. I haven't seen any photos online of this plant growing like mine, should I have been guiding it's growth? Is it going to die because the the main stem is so dry? Should/can I propagate the healthy newer stems? Thanks in advance!
This is typical of aged trunks. It appears to be doing what it should. Here is an article that will help you to care for the tree:
https://www.gardeningknowhow.com/houseplants/pachira-money-tree/growing-money-tree-houseplant.htm