For what you are doing, I would go with either a genuine a Sinadder (if you can find one), or the the Sinadalizer from the UK, which is pretty much a clone of that circuit.
I have an original Sinadder 3 here, and I prefer to use it over the built in Sinad measurement in my Marconi and HP test sets. You just can't beat that analog meter for Sinad tests, IMHO.
A voltmeter doesn't do what a Sinad meter does. A voltmeter will show a total AF level, including distortion (i.e, how loud it is). A Sinad meter will allow you to see when your RX sensitivity is best, using a built-in 1KHz filter to allow measurement of the distortion and noise in relation to the desired signal. My Sinadder 3 aslo has a voltmeter mode, so you can use it both ways. The built in speaker of the Sinadder 3 is also nice to have.
Motorola also made a nice little Sinad meter back in the day, an R-1013A. Keep an eye open for one of those too.
73