You could get along without the chisels if you punch the holes with a stitch awl. I've been working leather for 50+ years and never had a set of chisels. You can make a good, even line of holes with a bit of practice. I learned to stitch in a saddle shop and we just learned to use a stitch awl. There are good books on saddle stitching that can get you going in the right direction. Then, if you wish, you can invest in some chisels but it will b e a choice, not a necessity.
I used to have a stitching pony but it never held the work tight enough without marking the leather so I finally got rid of it and just work without it. FWIW, we never had a stitching pony in the saddle shop, either. You may be able to find one that does the job, though.