You dont need a pressure pot, but they do make things so much better. Casting under pressure eliminates a good amount of bubbles in the final product, and if you get a sheet of 1/2" polycarbonate, some fittings and a vacuum pump the pressure pot can pull double duty as a vacuum degassing chamber. Bubble free, optically clear castings are a beautiful thing.
I dont doubt that a cheapie silicon mold off amazon will do for a few castings, though i am skeptical that theyll hold up over time. That said, for a simple mold, properly cared for with a slower resin, youll probably outgrow the quality of the mold before it wears out.
For price-conscious resin, and at risk of sounding like a brand rep, id look at the trial kits for Smooth-Cast: https://www.amazon.com/Smooth-Smooth-Cast-ColorMatch-Plastic-Compound/dp/B004BNC9R4/ref=sr_1_4?keywords=smooth-cast+326&qid=1580642602&sr=8-4&x=0&y=0
Doesnt break the bank and its easy stuff to work with, just mix equal parts by volume and pour. Be warned, the 325 has a 3 minute pot life, meaning you have 3 minutes to mix and pour the resin before it goes all angry jello on you. Great for simple, open molds, less great for complex enclosed molds