I don't know much about leather but I do have a pretty good working knowledge of old domestic sewing machines.
The Singer 99 is a personal favorite of mine. They're sturdy and well built ,little machines BUT they were never made to sew thick leather. They're fantastic little machines for kids to learn to sew on though, especially when you fit them with a hand crank.
If you're serious about leather don't bother with domestic machines. Even the better ones like the Singer 201 or 15 really aren't designed for anything heavier than garment leather. Most don't want a steady diet of that either.
Not all commercial machines are created equal either. If you go for a used commercial machine make sure it's designed for heavier materials. A machine set up to sew ladies underwear isn't likely to work well on heavy leather. A lot of Craig's List and Ebay sellers like to claim their machines sew leather. Verify the model and the machine's capabilities for yourself before you buy.
ISMACS is a great resource for researching old machines. Here's a link to their section on Singer industrial machines.
http://ismacs.net/singer_sewing_machine_company/singer_industrial_model_sewing_machines_and_accessories.html
It should at least give you an idea of what you're looking at.
Rodney