The device you cited is spendy; if your budget does not permit acquiring it, there are alternatives.
You could get an iPhone and the xSensor app cited in this Wired article, or an Android phone with three-axis magnetic sensors (note: Not all Android phones do have three-axis magnetic sensors) and an app like Physics Toolbox Magnetometer as those apps measure the ambient magnetic field on all three axes. Then, log the results periodically.
I'd also suggest determining the accuracy of those historical records as a guide to how precise the measuring instrument must be. If the records you use as a baseline are not very accurate, spending a lot of money on a very precise meter may not be very useful.
The ARRL does confirm terrestial magnetic fields change frequently, BTW.