You are not going to mess it up. Why be frustrated with a 20.00 machine. You yourself said how nice it was to sew the pants and not be frustrated. If a machine is a pleasure to use, you will use it and it won't just sit collecting dust.
I'm thinking your 158. prefix machines were made by Jaguar for Sears. Don't hold me to it though.
I'm searching for the chart of manufacturers prefixes. After the 158. prefix 385. is the next manufacturer prefix which is Janome.