AllATP's have a good reputation. However, it is intense. You should be fully prepared ahead of time for the writtens, and if really on the ball, have them done before training.
You can avoid a couple by combining (i.e. Instrument and Instrument Instructor as one exam).
As you know the program offers a lot of Multi time (Seminole). The three hrs right seat, "SIC" jet is IMHO not likely to mean much in your logbook - but the experience will at least give you some background to turbine ops.
As for purchasing a plane and flying with an instructor, you could buy a Seneca or Seminole (look at insurance before you do) and fly your nads off for a few hundred hours then sell it. Be prepared to cough up a lot of cash for maintenance (did I mention Insurance?).
But to do the above, you obviously need the ratings (Private, then Multi).
If I were you, I would investigate AllATP's at Riverside first hand.
BTW I am in N Cal, and have NOT attended ALLATP's. Just thought about attending myself at one point, and got a lot of gen on the operation....
Blue Skies.