Save up some money and buy a machine from one of the dealers that has and ad at the top of the page. With any of them, you can tell them what you want to do with it, and the machine will pretty much come to you ready to assemble and sew. This is not the same as buying a machine head off of Ebay, and then having to get everything 'just so'. The machines from our dealers are modified (motor and reducers) to specifically sew leather. When you order one, they will be 'sewn off' before leaving the dealer. That means that the basic tension settings will be made (according to the thread type and intended thicknesses you'll be sewing), and all adjustments made so that when you unpack it, bolt it together, and plug it in, you'll be good to go. You will of course have to check the tensions for sewing the particular pieces you use, but that's true with any sewing machine. You WON'T have to try and modify the machine, buy a new motor, add another balance wheel, grind off some metal to increase the presser foot height, etc. Don't just look at the websites, CALL and speak to the dealers in person and explain what you'll be doing. This gives them the info they need to best serve you.
Unless you are an experienced tinkerer with lots of tools and time, -or- an experienced sewing machine mechanic, you'll do a LOT better for yourself by getting the right machine at the start. Not trying to be mean about it, just trying to convey the answer to a question that is often asked.
Here is a link to a thread in our sewing machine sub-forum: http://leatherworker.net/forum/index.php?showtopic=25239. This should help you decide on the type of machine to get.