A couple of points for consideration.

As it’s test equipment, you really need to use high-quality components so that you get repeatable readings after a few years use. Caps are one of the things you don’t really want to use budget versions of. F&T or similar quality.

And getting the component layout right is as important in a tester as in a valve amp. You don’t want the circuit to induce noise into the valve if you are using it to measure the noise levels of valves.