I’ve had good success with old Kenmore machines from the 60’s and 70’s that were made by the Japanese Maruzen company. These start with the model number 158.  You can often pick these up for $50 or less.  These machines have a cast aluminum case and all metal internals.  The best ones to get for heavy duty work have a 1.3 amp motor and a double reduction drive (two belts) that increase torque. The top of the line models (1803,1813,1914,1913, etc.) also came with a huge set of accessories that includes a bunch of different presser feet, a button hole attachment, and even a monogramming attachment. Often you can find the machines complete with all the accessories, manual, etc.  I’ve successfully sewn through pack cloth, webbing, etc with these without issue. I currently have two – a 158.1804 (flat deck) and a 158.1914 (free arm).
The other machines to consider are Japanese HA-1 type machines. These are zigzag machines with a cast iron case. They came under several brands (White, Brother, Dressmaker, etc), but can be identified by the fact that they have all metal internals and are super heavy (due to the cast iron case). These will also sew through tough materials quite easily.
As Ken said though, like Necchis, they are all still home machines so they’re okay for occasional heavy use but if you want to do this kind of stuff frequently, you might want to consider getting an industrial machine.
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