Spoiler alert: you are on a tight budget and have minimal knowledge working on a car. A custom transmission is so far out of your price and knowledge range don't waste your time thinking about it. You bought the wrong car if you want a manual transmission.
For the coolant, just do what Scotty says and you'll be fine. When you are refilling the coolant, you can't instantly fill it full - it takes some time for the coolant to flow into the whole system. That is why you'll have to keep topping it off.