Replaced engine and transmission with used

This might be long, cause Im gonna give alot of details.

I have a 2003 Tiburon gt v6 Automatic with Side Shift.

I bought a used 2006 Engine with about 120000 miles on it & a Used 2006 Transmission with about 120000 miles on it as well, both from different States an shipped to my Repair shop that I choose to do these repairs.

$2400 later, Im given back the car with a Huge Problem that makes me seek out help, answers and another repair shop with another Bill, Im not sure how Im gonna afford.

Repair shop came in my car to pick me up when he said it was ready. Drove an felt fine for a bit, and was looking good an feeling great, sounded smooth, ect.
Then Im told I have to follow the following steps now because if I don't then a problem will arise an I'll end up having to call for a Tow or hope I can rock the car to get the Shifter to move from Park into the gear I need to Drive or Reverse, ect.
#1. Getting into the car, I put foot on break pedal, move from P to R, then let the E Break off, an R the car, come to stop with foot break, then Pull E Break again, then move from R to D, then let E break off, then let foot off break an hope that it drives an stays in gear.

#2. Parking the car. Come to a stop, Pull the E Break, then Put car from D to N to r to the Park! Once these steps are done, slowly each gear, I then have to Pull the E Break again, an turn car off.

#3 If I do not follow these steps carefully each time, I risk *(and it happened while repair guy was showing this all to me) When I try to get the car to shift out of Park, It wont want to move at all.
(while this took place, the repair guy asked his helper, to get in the car, an try to move the shifter. It didnt wanna move an I could see that he was pulling Really freaking hard on it. *I was thinking to self, this isnt right, its not safe an this guy is gonna break something more* While the guy in the car was trying to move the shift, all of a sudden, the shifter slammed through the gears, however, I could tell that the way this happened Was Not Normal and sure to cause a problem inside something, someplace. (Idk, Im not a repair person at all) Then the repair guy outside the car was under the hood, he was trying to rock the car back an forth to Unstick something, I cant remember what he said.
Once the car was able to go into R for us to leave the parking spot, we then took the steps an followed them to Drive it back to the repair shop again.

*Main Repair guy that owns the shop, tryed to tell me that my car had Recalls on the Shift Assembly for my 2003 year. And that I needed to find a replacement one for it, cause he couldnt.
*Ive searched online called places cant find any.

So now, we move into the Next Issue that is happening.

While Im Driving the car, at under 40MPH an at 20MPH this has happened at least 5 times now, and Im scared to drive her at all again, (At this point Im in a total og $3400 for all this replacement parts, labor and supplies ect, and still dont have a working car)

While driving, I can see on the Dash the letter D as it should be. Then suddenly, the D on the Dashboard goes out, and the car isnt moving forward as it should, and the RPM go up due to the gas pedal being pushed. I put hand on Shifter to see if it has moved at all. (Not from what I can feel it has not. And it did not feel like the tranny was jerking out of gear at all either, this happened so smoothly) Then I slowly pres the foot break an start to pull car to side of road, An then, The D on Dashboard comes back on again, and its back in gear as nothing happened.

I contacted the repair shop and he didnt know why it happened an told me that it didnt happen when he had it.

After not finding a used Shift assembly anywhere online for my car.
I found a local place that deals only with Hyundais. I called them an spoke to a guy there. And was told that he would go ask the Service Repair guys in the shop there, but he couldnt promise me anything cause normally they gonna just say to make a time slot to pull it in.

Explained my situation with having a $2400 bill I have to pay small monthly payement already, and only having so muc left over each month, Im not able to just take her into a shop, let them do whatever an be told a bill of an amount I cant have upfront an risk them not taking payments, ect.

So. I do not have a list of replaced parts that had to be used in the first shop, cause I cant seem to get them on the phone or to get a list made up. *This should have been done while they had my car, doing the work I feel.

Ive been told it could be.
#1 sold a bad Transmission
#2 the Computer software didnt get updated at first repair shop (No the first repair shop didnt know that it needed updates an didnt do this) Cause my car is an 2003 an the used engine an tranny are from 2 different cars from 2006
#3The Transmission was Alined correctly the first time, is what the Repair shop guy told me from the first place.
#4 Maybe Broken Transmission bands (would the first repair guy have known this while installing it?)
#5 maybe damaged torque converter (would first repair guy have seen this too?)
#6. Bad Transmission Solenoids (would first repair have seen this too?)
#7 Shift Cables poss worn out due to age and poss that the Engine mounts cauing it to pull on the cables (could this have been caused by when the repairs guys ruff pulled on the shifter to move an forced it out of park and then it seemed to me that the shifter then moved up an down the gears in the car while in Park.?) *Keep in mind that the D going to N didnt happen until After the Repair guys pulled the shifter with force before hand.
#8 is it poss, loose/out of adjustment shift linkage on top the Transaxle could be the reason?
#9 Maybe its a Bad ground of computer that is going out an needs a pin to pin check?

Also, forgot to tell you that the Engine Mounts did all get replaced with New ones upon this repair job.

I'd also like to add, that I do have a stereo system in my car that is pushing two 12 inch subs, and two Amps (600Wtt & 1200Wtt) an it has been in the car for about 13years, an Does rattle the car when played on high, even enough to poss wiggle something lose over time, maybe too.

I truly hope that this is enough details, but not to much, cause I really could use some answers, an Im hoping that after its taken me 1 hour to type this up on my Xbox, that its not for a waste of my time on this Message Board.

Thank you for your time,

Oregon Tiburon car owner needs your help !