Since your motivation is to become a better shooter, I'd echo Steve in PA.
I'd likely pick the KelTec to start, since, unlike the j-frame, it can be loaded to division capacity.
Shooting revolvers in competition is an exercise in reloading skills, which would really be put to the test with a 5-round gun. Since you don't even own speed loaders yet, I see you experiencing more frustration than anything else if you jump right in with your 640.
I can wholly recommend using your 640 at some point (it would most certainly make you a better shooter), but I'd buy some speedloaders (CompIIIs or Jetloaders), holders, and dummy rounds and practice your reloads at home first.
Max range? Depends on the match. Most shots are 7-10 yards. A long-ish shot could be 15 yards. Though rare, a 20-25 yard shot occasionally shows up at one of our local matches.
A .45 1911 for IDPA? It's an excellent choice - so much so that there's an entire division largely just for them - Combat Defense Pistol (CDP).