a few things to bear in mind -
Interface thin leather (it stretches)
Soft and slightly thicker leather is great for bags but think about seam types and construction, how to reduce bulk and edge finishing.
For Edge finishing investigate 2 methods - edge paint and skive & turn. Lots of info around and on you tube about those.
Skiving helps to reduce bulk. There are some other useful methods for reducing bulk (and spending on hardware) for example this method of making
hidden strap connectors and when making straps with leather, try leather on one side and fabric on the other. Also make friends with glue - not everything needs to be stitched.
A Tip - if it's going to be a one off, skiving (shaving to thin down) takes a bit of practice and you probably don't want to buy a skiving knife unless you expect to do several more leather projects. For a first project, try working with a thin leather and edge paint for finishing, plus you can use patterns designed for thicker fabrics.
Sofa leather can be great for bags but it is usually in the medium to thick leather.
choose with care the style of bag number of seams etc
For hardware, if you're not a prolific bagmaker, go with chicago screws rather than investing in a double cap rivet set. If you're going to include zips, get some bag making zips, they tend to have bigger and nicely shaped pullers - the most common use sizes are 3#, 5# and 7#
the thicker hard leather uses different techniques ie. more in the zone of leather work techniques, so maybe leave that until another time should the bag or leather working bug bite.
Finally, there is an excellent, friendly and helpful bag making group on facebook with information re suppliers and recommendations -
https://www.facebook.com/groups/britishbagmakers/?ref=bookmarks . It's also the parent group to several regional groups and some of those meet up from time to time.