Meters like the HP400 series are particularly useful when trying to adjust things to peak a signal level. Much easier to see the peak than on a digital multimeter. Whether the HP400 is more accurate than your Sanwa, the answer is probably yes. Typically, HP analog meter accuracies are in the 1-2% range (of full scale).
The meter is easy to use. Just set the range switch until you get a reading. The bottom scale is for the "3" ranges (0.3, 3, etc) and the middle scale is for the "1" ranges (0.1, 1, 10, etc.). The top scale is a decibel reading and is referenced to 600 ohms. In other words, about 0.773 volts through a 600 ohm load will give you a 0 dBm. You can pretty much ignore the top dBm scale if you like. The meter has a 10 Mohm input impedance and should be able to measure things without loading the circuit down.
These meters don't have overlapping scales. By that I mean you will find there is only one range where you will get a valid reading. The other ranges will either move the needle all the way to the right or to the left.
This is an average responding meter, not a "true RMS" meter. In other words, for any signal input more complicated than a sine wave, the readings may be incorrect. But again, if you are using the meter to make adjustments while peaking a signal, it is fine.