Most of what you listed are items you should have.
A headstall...pretty basic necessity and not expensive if you don't want it to be.
Same as a snaffle bit...can purchase that for around $20 in many styles, variations and mouthpiece size. You don't need to spend $$$$ for a simple snaffle bit although they are out there.
I'm not a TT lover by what I have read and what I have experienced on some horses I have ridden in the past...what works for some, may not work for others is true for horses and their individual like and dislikes...this bit still doesn't need to cost $$$$.
Side reins... if you are going to buy them, try out the different styles and find what you like and are comfortable using. Then buy not the cheapest but something better quality.
A backpad...as in harness pad or a saddle pad?
If you aren't buying a harness maybe hold off on the back pad since you need it to be the correct size to your harness straps...
I don't know where you live, but do some internet searches for these items.
I would buy new as they aren't that expensive honestly and why buy a possibly half worn out item for almost the same price as new.
Go to tack swap meets and "horsey" yard sales... amazing what people will sell and for what price.
For a headstall, bit and side reins... I could purchase all of them new for less than $60.00 in good quality.
A complete driving harness (not leather) for less than $200.
Leather harness driving reins (long lines) for less than $50., in nice quality.
Yes, I could also spend hundreds for each item but why do that if you are only starting a animal out and it may be destroyed/ruined or not work...makes no sense.
You need to look and search for good products at reasonable prices. Unfortunately, if you buy everything through a tack shop you are going to pay so much more....hence know what you want in style then go look for it online and buy it from reputable dealers who stand behind their products and have been in business for years.
I don't do Ebay myself, as you don't know what someone is unloading on you and if it is as represented or is junk...and you have no recourse from private sellers like you do from a business.
Hands on look-see always if you buy any used leather to make sure it is in usable condition and not cracked or stitches falling out, missing or garbage.
Good luck in your search for equipment.