You need to save up at least $1000 for a decent used leather machine, I will explain. You will be tempted by the "great deals" that can be found on Craigslist. Sometimes they are great deals however buying a 50-100 year old walking foot sewing machine can be tricky. Most of the parts are probably good but you may need a few new parts, and you will need the correct needles and thread (they can't be found at Jo-Anns). This process is not unlike getting a classic car be prepared to invest time and money. I have found that used machines typically cost 2x the purchase price to get in working order. So if you pay $300 for a Singer walking foot machine like a 153, or a 111 expect to pay up to another $300 to make it work perfectly. Typically there are plenty of parts however they are not all cheap and then you have to figure out how to replace the part and then readjust the whole machine, sometimes without the manual. The next issue is the old used machines typically come with a clutch motor. The clutch motor while super strong and fast is a nightmare. Sure there are tricks to slow it down, swap pulleys, add pulley speed reducers, add bungee cords, it will drive you nuts. You lightly tap on the pedal and zoom the machine takes off and your project looks like Frankenstein. So you will need a servo motor. They will give you control over your project. So now you have your $300 machine that needs a new bobbin case ($40.00), new servo motor ($150) , few packs of needles and thread ($50) a six pack of beer for your buddy who helps you carry all this ($10) since it weighs almost 200 pounds. Now your up to $500 and you still have not sewn anything.
I recommend looking for a machine that already has the key features you will need, servo motor, extra bobbins, needles, bobbin case (shuttle), thread and extra feet attachments. Bring a few test scraps of leather with you so you can test sew. If it is set correctly you should be able to walk up power it on and sew a line without issue. If you hear anything grinding, the thread getting tangled in the bobbin or breaking, the stiches look bad or very loose I would walk away and keep looking. However if it has all this then you should be in good shape. Bring a flashlight with you so you can look the machine over. You can't see all the little parts without good light. If your missing a few tiny screws in the wrong places the machine will kind of work but not exactly work well. They you will be so angry when you have to order .89 cent screws and pay $10 in shipping to get them.
Also because this bothered me when I started- Why do they all look the same?
The new machines are practically all based on the old machines and this is why they all look similar. If you look at similar model lines from Consew, Techsew, EconoSew, Cowboy, Juki they are appear at first glance to be the same machine re-branded. Adler and Phaff also fit into this but they have a few features and parts that are unique to them. All of them in the correct model machine and configuration servo motor can be setup to sew leather, plus the have probably have reverse. Just like the 75 year old Singer they can run almost forever. However because they are new or slightly used they will not have the issues I described above with the 75 year old Singer. You get a clean machine that is ready to sew. Expect to pay $800 plus for a used system like this, up to 4k for a new one. I can't recommend a one brand over the other, what I can recommend is that you visit you local industrial sewing store and see them for yourself. This will also help you narrow your search because you may want a post machine over a cylinder arm machine, they will have all the machine variants to look at.