If you are mechanically inept, I highly suggest you head out and buy yourself a complete single speed from any number of companies. Better yet, buy yourself a used one from a hipster who bought it and rode it 5 times before realizing that he/she liked gears. Or from a guy on Craigslist who has already done it for you. I am being a little harsh, but you will spend MUCH less $$ buying a pre-converted bike.
To buy components for your frame, you are talking in the neighborhood of $200. Possibly $150 if you are a savvy shopper. You will then need to install all of said components, which will require a full set of tools.
That said, to get a new crank, you will need a new bottom bracket, which you most probably would have to get anyways. Especially since yours is for a cottered crank, which you don't want to mess with. And probably couldn't find.