You will need to get around 3,000,000 gold in order to buy every weapon and piece of equipment.
You will get gold from beating arcade and story mode, winning ranked matches, and completing levels for Tower of Lost Souls. To save money, you can unlock items by completing certain tasks in the TOLS levels.
In order to buy all pieces of equipment, you have to unlock a certain number of achievements (I believe 25 or 30). Accessories (items for the guest characters, Algol, and Star Wars characters) might count towards this achievement, but I can't confirm that. A special note: Voldo has a piece of equipment that can only be bought when editting him or a character modeled after him. You have to buy all equipment for both genders as well.
In order to be able to buy every weapon for each character, you must beat the game with each character in story mode. Pretty much, beat story mode 32 times (one time for each character).
There's an exploit to gain some fast gold. On level 60 of TOLS, simply walk back towards the ledge and let Algo knock you down. When de you're down on the ground, don't touch the controller. If your character is angled correctly, Algol will ring himself out and you'll have 12500 more gold. Repeat as many times as necessary.
To make things easier, organize how you'll buy things. Set out to buy all male equipment at once (on Voldo), then all female equipment, then buy all weapons for each character one at a time (So, buy all the weapons for Siegfried, then all the weapons for Nightmare...etc etc).
EDIT: Since Izibaar requested, it should be noted that going through the story mode with every character and completing arcade mode a few times will net you around 1,000,000 gold. So you can get a decent amount of gold in your bank (as well as all of the weapons unlocked) before you go about doing the TOLS exploit.