I have converted violins to have a fifth string tuned to C on the bottom. Works nicely since the lowest string has very little tension. A violin G string or a viola G string works well.

As for cheap violins, I have Chinese fiddles that out perform French or German. The most important carving is inside on both plates all around the edges. If they are thin enough to freely vibrate along with ribs that are also extremely thin the fiddle will sing with double the intensity of other fiddles. The thickness in the center of the plates must be maintained along with some variable rules regarding tap tones at certain zones.

A hundred years ago the Virzi resonator was an add on to enhance vilin tone. I created the soul post which exceeds the Virzi by enhancing the middle and upper ranges, particularly on instrument that already produce good deep tones. A cubic inch of the material weighs the same as 12 long human hairs. The placement is roughly the same as the human heart in the upper left hand area of the violin or cello.

What not to buy -
Any fiddle with squarish upper shoulders or a violin without big round hips... with the exception of a nice small Guaneri.

Currently I am satisfied with a baroque N Amati cello that was around when JS Bach was still alive. It has been reinforced to take steel strings but sounds as golden as its current gold birdseye maple color. It also lost the original wood on the right shoulder and lower front but has been so that only the shoulder is noticeably smaller when scrutinized.
We all get a dented fender in old age.