Tegrak lagfix: how do I install it?
Load the kernel .zip file into the PDA field of Odin, and (if you have it, should have come with Odin) load the 512 .pit file into the PIT field of Odin.  Then flash it just like you did for the Froyo ROM.

A stock ROM is just a ROM that's not modified by anyone, it came straight from Samsung.  If you aren't using a lagfix already, you probably don't need a stock ROM however.  You should be able to find one easily enough in the XDA subforum for your device, if you want it; use JS5, since that's what the kernel is based on.

Your paid apps are recorded in your Google account.  You will never lose your purchase rights to them (barring any service suspensions and that sort of thing).  You can, however, use something like Titanium backup to back them up (requires root).  You can back up the entire device using ClockworkMod / Nandroid; look for ROM Manager in the Market (requires root).

One question: Why are you using that over straight Voodoo?  Voodoo's perfect, and I believe Supercurio has taken stock kernels and added only Voodoo so you don't get any other random stuff.