Look for an old Bernina or Elna (preferably from the 1960s/70s). Those are pretty well bombproof machines with all metal parts and exquisite engineering. Kenmore machines from the 80s are usually good too. Stay far away from any Singer newer than forty years, any Brother, and the off-brand machines that go on sale in every city at least twice a year as "school sewing machines, lightly used." Nobody knows where those come from, and they aren't fixable.

Check your local listings for a Bernina dealer and start there. (I had a Bernina 730 that was fantastic).

Leather sewing is mainly a matter of specialized needles, by the way.