You would need a full set of Vegas boards (all 3 boards) plus the I/O adapter board. The I/O adapter board is used to convert the connector that Seattle used for the outputs to the I-40 board to the connector that Vegas uses. If you can't find the I/O adapter board, it can be faked with a custom wiring harness. If you only have 8-way joysticks or a 2 player cabinet (and hence no I-40 board), you don't need this, but a dedicated Blitz 99 setup should have 49-way sticks with an I-40, so if you aren't sure otherwise, you probably do need this I/O adapter board.
If you plan on buying the boardset separately from the game program (as in buying a Vegas boardset running something other than the Blitz/Showtime combo), you'll need to acquire all four items from the "upgrade kit": hard drive, boot ROM, security chip, and sound ROM. If the boardset started out running Blitz or Showtime, then you probably (but I cannot confirm) do not need the sound ROM and possibly the boot ROM, but you still need the hard drive and security chip. New upgrade kits are not generally available, but bootlegs are pretty common. They usually work fine, but they aren't real. Every once in a while, you can find someone with a NOS legit kit, but that's getting rare.
Easiest path is to buy a boardset already running the Blitz/Showtime combo. Then it's just plug and play as long as you have that little I/O adapter board.