The lenses already suggested are all good lenses but none are optimised for the task you have in hand. In your shoes, I would spend that budget on a Canon bellows unit and a second hand macro bellows lens. The lens need not be from Canon as bellows lens adapters are freely available and not costly.
Bellows lenses are optimised for reproduction at 1:2 - 2:1, have an almost perfectly flat field (essential to get sharpness across the whole negative) and have minimal chromatic aberration. General purpose macro lenses simply cannot do this as well.
You could look at Leica Macro Elmar 100mm F4, Olympus 80mm F4 or 135mm F4, etc.
Top quality enlarger lenses also work well such as Schneider Componon-S 150mm f/5.6 Schneider Kreuznach Componon 150mm f/5.6. Bellows needed and adapters are easy to find.
Best of all are line scan lenses if you can cannibalise from a broken scanner on ebay - Nikkor ED 100mm Scanner Lens from the Coolscan ED 8000/9000 or the lens from the Minolta Dimage 5400 (best of all). Again, a bellows and adapters will be needed but the results are spectacularly better than any other lens.
That said, the Sigma Art and the Sigma 70mm F2.8 DG are very good and outperform the Canon 100mm F2.8 which is disappointing by comparison.
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