Congrats, on the (potential) purchase!
For a single disc install you will have to physically toggle on DIP4 on the motherboard (the "573" system as it's generally called) itself. If you take the board out and look at it you will see that there is DIP switch bank near one of the edges of the board. I believe on a 573 it will look like a blue rectangle with tiny switches coming off of it that you can turn off and on. Each switch should be labeled 1, 2, 3, 4 and you will want to turn on the DIP switch labeled #4.
After you do that you just turn the game on and it will go into the install process and you'll install the game. The actually play the game you'll need to toggle DIP4 back off after the install process.
The 573 system will look like this:
As for the P2:44 error -- a quick Google search tells me that that error is related to Playstation 2 hardware (otherwise known as Python2 hardware in the arcade world) which is what DDR ran off of AFTER Extreme came out. Do you have an eBay link or a Craiglist post we can look at to help determine if everything is legit with this machine? Based off of that error that tells me that this machine might be a shitty Namco cabinet that's running a hacked version of Extreme USA or something.
If your intention is to gut this cabinet and put a PC inside of it to play Stepmania then the boot problem is sort of moot but if you're planning on playing official mixes then it will be a good idea to make sure to check what hardware you're actually getting with this cabinet.