This is a very interesting restoration project you are undertaking. The top looks like good wood with nice tight grain. The curved section or rib that was changed may not be the only change. Look close at the purfling (the inlaid striping that runs close to the edge of the top). It appears to run out on both of the shoulders of the bass. It looks like the shoulders were cut down at one time. This is apparently common with older instruments. It also looks like the top has several cracks (six places that look like cracks). If these cracks were previously repaired, you need to make sure these repairs are still secure. It is possible that the damaged rib could be repaired rather than changed if the damage is not too severe. First get rid of the old repair (carefully) and see what is there. Bending a new rib might require heat as well as soaking in water.
The height (15 centimeters or about 5 7/8 inches) of your old bridge is pretty normal for a 3/4 size double bass. The wood and grain of the cut look all wrong for a good bridge. It is an interesting looking bridge, but get a nice maple one with similar height. More wood at the top and a little less leg is more usual. What kind of wood was that bridge made of? It looks like rosewood or cocobola, or something exotic.
You are correct to be suspicious of the ebony FB at =$125. That is about what rosewood fingerboards sell for in the USA. The least expensive ebony fingerboards I can remember seeing were U$D 200. It is possible that it sells for less, depending on demand and source. Not all ebony is straight-grained or uniform enough to make good fingerboards. What kind of wood was the original fingerboard?
Alcohol (shellac) based finishes are the easiest to apply and repair. Oil is the most durable, but requires more time and care in appication. Good luck with the project. I look forward to the results pics, your shots of it in progress are very sharp and well done.
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