The Nokia CA-42 cable is well known as a cheap USB to RS232 converter. You can get them on eBay for about £1. One thing that was not clear from most of the articles about it is how tolerant the signalling voltage can is.
According to the datasheet for the IC used in the CA-42 (a Prolific PL2303HX) it supports both 3.3V and 5V signals. The CA-42 provides 5V and 3.3V and the PL2303HX outputs 3.3V on it’s TX line, but putting 5V on the RX line is specifically mentioned in the datasheet as being safe.
As for the power lines the 5V is probably direct from the USB port, less the amount used by the PL2303HX, so it’s likely to support a few hundred milliamps. The current limit of the 3.3V line is unknown but most likely in the 50-100mA range at best.