Welcome! While I understand the stress as well as the excitement, take a breath and relax. If you seriously want to go marine, I wouldn’t go for anything smaller than 40 simply because the parameter swings.
Gather your equipment first. You’ll need to make many decisions between gathering and setup, and you need to read the entire time. Look at build threads here for stocking ideas as well as good advice with mistakes made by others
Search Craigslist for a used setup, either the whole thing or just a tank. You can build a stand and a sump. If you have places like goodwill or thrift shops look there as well. It would be nice to go with brand new but sometimes a little used will save money.
Don’t completely write off fresh, there’s many varieties of fish that are beautiful and many you will never see in any life or even on live aquaria.
Read, read and read some more. Basic knowledge of the nitrogen cycle is needed for any setup, fresh or salt and you’ll also have to know about the salinity and how temp ph and other things effect such things.
For me it also helped to write down what my end goal was, and do a price list for the equipment, fish and supplies.
Never try to rush through or you’ll end up hating it. Take it one day at a time and make it fun. It will reward you eventually.