Looking back at experiences when I was learning, 35 years ago, I've never been comfortable with wacking a cotter with a hammer, no matter how well supported. Once I got back into wrenching, a cotter press was my first expense. If you seriously considering keeping the bike for a reasonably long term, spend the money and get the right tool.

Quick, cheap substitute for a one shot removal: See if any of your local bike shops employ a mechanic who's middle aged. If so, he's probably been wrenching for decades and has the tool in his kit. It won't cost that much to have the cotters removed properly. That's how I got my first bike going back in '05 while I was looking for the proper tool. It was the start of a good friendship, too.