There are specific old Singers that are more heavy duty than others. Can't remember which ones offhand. To find out, join one of the vintage Singer sewing machine groups at
http://groups.yahoo.com . Someone there would be able to tell you which models are good for sewing sails and leather.
The pros of one of these older machines is that they cost a lot less than new ones and you can maintain them yourself. It's fairly easy to find replacement parts for them. The con is that they are harder to find. It can take awhile to hunt a good one down.