front end wreck repaired. Won't shift into D or R.

Sorry, I posted this last night but can't find my post nor replies to it. I'm new to this site. Hope this post works. My 04 Prius was in a front end wreck, and having no comprehensive insurance I did the mechanical work myself - radiator, fenders, hood, inverter, inverter coolant pump. Still looking for the air filter box, 2 airbags (strg wheel and rt dash), and 2 front seatbelt retractors. Questions: Car sat around maybe 3-4 years. Probably half tank in it. Could be stale? I installed the aux battery from my 06 Prius, and charged it up. 04 Prius won't engage D or R. It goes to P and N ok. Upon boot up, I get the "ready" light for 5 seconds, then it goes out. If I disconnect the aux battery, and leave it for a day, the first time I boot up I think the ICE engine will try to turn over very briefly (1/4 second). It won't let me do that again, unless I pull a battery cable and wait overnight (or at least a few hours, I surmise). When I try to shift into D or R, I sometimes get the error "transmission park error...park on flat surface and set parking brake" (or something) on the MFD (I heard this was due to a weak aux battery). I get the red triangle, and the check engine, brakes, ABS, and skid lights are on. Sometimes, I get the red "car cut in half" light on the MFD. Before I tow it to the dealer with a blank check, is there an easy fix? Should I install a brand new battery? Should I check the brake pedal switches (I heard there are 2)? What else can I check? Fuel pressure? Check if gas is stale (how?)? Remember, the airbags & seatbelt retractors & other SRS parts are still deployed and haven't replaced them yet (need leads on cheap new/used ones and to decide which other parts to replace). I heard the car should run w/o these replaced. I think I need an independent tech with a Toyota scan tool. I'll consider all leads?