When it comes to sewing machines; I'm not so great. So I'll try to keep this in the spirit of 'bespoke tailoring' as best as I can while not really knowing what the hell I'm doing.
For most sewing projects I undertake I like to do as much by hand as possible, so I'll list some of the tools I generally use
I have about 600 quilting pins in a bobbin case that end up getting used for just about every project I have ever taken on. I recommend the quilting pins that are about 2'' long since they are much easier to see and seem to get you a bit further than the 1'' pins. It's doubtful I still have 600, but I did buy a box with that many in it in the past few months. These things will unfortunately leave the box and end up everywhere when you sew somehow. Note to self: Get a pin cushion!
I like to use longer needles as well. The needles I use might be some sort of heavier quilters needle? Unfortunately I'm not quite sure of this as of this moment.
I'd like to get a thimble, but generally I just use smaller pieces of leather scraps from old projects since they seem to do the job well enough.
Having a vague idea of what you are doing is very important, so in order to fulfill this little part of sewing I use a square with a grid of measurements when I can't use a measuring tape. I know there are more tools out there for measurements that I'm undoubtedly going to need while I try to learn about this, so we'll add those to the tools as we discover what all we are missing.
Scissors are definitely nice to have. Using your teeth to shred fabric and thread probably isn't the best idea, so I would recommend getting some scissors. There may be better scissors out there, but I like to use Wiss steel scissors for any fabric that isn't synthetic.
Below is my basic sewing kit for many projects; pins, needles, grommets, measuring devices, scissors, 'saddlers palm,' and a spool of artificial sinew I use when working with leather.
Of course other tools are nice to have that I haven't mentioned. Machines are great, but I'm still learning about tension settings and dealing with the frustration that causes. A 'hot knife' is incredible as well, but they aren't so nice when you are cutting leather or natural fibers.