Dont buy a modchip, first off, you probably wouldnt be able to install it unless your are really good at soldering
And right now modchips only either 1) let you have 2 firmwares on the psp 2) if you brick the psp you can plug in the USB and reflash it
So with that, your probably better off just flashing your psp with custom firmware cause it ends up doing the exact same thing since your going to be flashing your custom firmware on a chip anyway
The only thing the chip is really good for is to make your psp pretty much unbrickable
Yes, you can back up YOUR UMD's by using the newest filer, or a app. called USB SSS. I prefer using USB SSS because its easier