All you really need is some way to cut out the leather, and an awl, and some needles. Your OP suggests that you just have one project in mind.
- If you are trying to build a leather working toolset, then it is a different mater. If I am planing on doing a hobby for my lifetime, I try to buy serious tools. Sometimes I buy best quality, when they are available, sometime I just buy pro quality, but that is almost more an aesthetic decision. I'm trying to make the best stuff I can, and that can go hand in hand with best quality tools as an experience. A skilled set of hands can make nice quality work with minimal tools, but better tools will save time and money over the long haul.
- Both skill and tool wise, entering a new craft is affected by whether this is the first craft you have ever done or not. Many steps, like laying out; skills like sharpening; and tools can also overlap. If you have a deep well to draw on then you can often get by without other tools. You may have a vise that can be pressed into service to hold a project for hand sewing. I have several lathes, that can not only make tools and components, but can quickly be converted into edge finishers, buffers, etc... I have spray guns that can shoot finish, computers that can print out drawings and transfers. Just to take a few examples.
- If you are close to places that sell leather working tools, Tandy is not so bad, then you can simply buy tools as you need them. You are right, there can be a lot of tools in leatherwork. If you really want to get set up, I would recommend getting a membership at Tandy, at least for the first year if you plan heavy buying. Also, it helps to never buy anything that isn't on sale. At Tandy almost everything goes on sale sooner or later.
- You can build an impressive list of tools from the projects themselves. If you are working from a plan, you can simply go through the plan and itemize ever tool referred to. You can do the same visualizing a plan you will be developing for yourself. Or keep score when you watch videos.
- Now comes the fun part, figuring out what you really need, and what you can work around:
1) For instance, let's say you saw a plan where the maker used dividers to lay out cartridge loops. Great tool to have, but most home computers and printers can print out extremely accurate patterns using simple cad software, or drawing programs. You can even make transfers that will allow you to print these patterns directly to the leather. The computer I use to do this is a desktop that cost me 69 bucks refurbished...
2) Another approach is to vary plans. If a plan calls for rivets, you can sometimes do a better job substituting sewing, therefore no rivet setting tools, or punches. Conversely sometimes (not necessarily holsters) you can work a plan that has a ton of hand sewing, into an even nicer one that uses rivets. Or sometimes you can avoid rivets with an upgrade to Chicago screws. Point being the design will determine what tools you will need and sometimes you can improve a design by substituting something that also uses less tools. And obvious example being hand sewing with linen thread can be an upgrade on traditional leather work, and save you buying a sewing machine. With carving you can choose designs that rely on the swivel knife and basic stamps to create unique work, compared to work that is all based on pattern stamps.
3) Another way to save on money is to borrow techniques from other crafts. For instance shoe makers don't use needles, they use wires or monofilament. This method is easier to do since the "bristles" pass through the holes with less effort than needles, Or allow the use of finer holes, where that might be desirable. You can save money on awls and sewing punches by using your drill press to shoot holes.