Most of the machines listed here won't do zig zag. How thick do you think you will be sewing (total thickness)? The Sailrite machines may work if you only sew thin stuff. Most people here will recommend something like a Consew 206RB type of machine. Flat bed, will sew a 40 or 69 thread which is pretty small. Keep in mind that people on this board tend to lean towards heavier leathers so their machine suggestions tend to be heavier duty than you may need.
For a flat bed, check out a Consew 206RB. Any of the subclasses (1-5) will do fine. There are better machines in that class but they are more expensive and the Consews will sew just fine. If you buy used, you may be able to get it with a servo for $600 but that would be a pretty good deal. I've seen them more in the $800 range.
If you can spend more money, check out something like a Consew 227R. Basically the same machine as a 206RB but with a cylinder bed which can be useful for sewing tricky gussets in bags. Flat bed attachments are available for them so they can work like a flatbed machine. More versatile but again, more expensive.
There are more expensive options out that there are better machines but these machines are already very good and their prices are going to be lower. Check out the dealers that are linked to on this board. They all sell either these exact machines or their own versions of these machines (basically the same thing but rebranded). Their service is good and if they have used machines, the prices won't be bad either. Cheapest prices will probably be found on Craigslist but then you'd have to know what you're getting yourself into. With these dealers, you'll know that you're getting a working machine with support behind it.
Andrew