Originally posted by pentkon52 Get a used istD. Dave
I think Dave got the right answer, perhaps for the wrong reason. An istD is a good choice if you are going to use it UNMODIFIED. The older cameras like the istD and K1x0D have a "weaker" internal infrared filter thus are better for IR.
The Original Poster was asking about modified cameras, and I suspect the newer cameras are just as sensitive to IR with the internal filter removed. Spencer's web site says they will modify most all Pentax models. The decision comes down to cost and convenience. My wife's istD is easier to handle, has two dials, and would cost less on the used market (it cost me over a grand
new). You may pay more for the conversion than the body!
If you have intentions of using a filterless camera for normal color (with the proper external IR/deep red blocking filter) than you might want a newer model for all the features. Myself, I use my filterless K110D solely for IR and Astro. If I was desperate (eg. my K100D broke down) I could use my Baader astro IR/UV filter and with a lot of color correction get close to normal color.
P.S. I just looked over Spencer's site and realize they add an IR filter making the camera dedicated to one function. Perhaps they would return a filterless camera if asked. Spencer's mentions sending all future repairs to them so the IR feature would not be disturbed. This may be wise, but I had no problem with Pentax repairing my K110D after I broke it removing the internal filter. I sent them a note, stuck a "filter removed" label on the body, and they fixed it without putting a filter back in. I was afraid they would balk at returning a camera that obviously failed many calibrations. The cost of the repair was under the cost of Spencer's conversion. Since my main objective was astrophotography an internal visible light blocking filter was not desirable.