You really want to get the horse(s) first, and then buy the tack. The reason is, not all saddles fit all horses, and not all horses ride well in all bits. So you kind of want to get the horse first, and ask the owner what kind of bit the horse rides best in.
Same with the saddle. Some saddles are narrow and some are wide, and it can be a LOT harder to get a good fit that you would imagine. Heck, even if you own the horse already, you can have a hard time finding a good fitting saddle. I think I went through literally 10 different saddles until I found one that was a good fit for my Mustang!
So, I definitely wouldn't buy the bit (although you can purchase a headstall and reins) until you find out what your horse needs.
And saddles, well, if you get a good deal on a used one that fits YOU, then you can take a chance on it, but you have to realize that it might not fit the horse and you will need to sell it and get a different one if it is poor fit.
The good news with saddles, however, if you buy good quality used
, you can usually get your money back out of them.
So, if you want to purchase some items in anticipation, that is fine, but try to buy the items you can use on any horse, such as saddle pads, reins, headstalls, halters, leads, etc. If you get a good deal on a saddle or bit(s), just realize when you get the horse, you might need to purchase a different one if it isn't the right fit for the horse.
But most of us horse people have a lot of extra tack laying around anyway. You never know when you might need it.