Congrats on the new purchase.
One of my rules of used car buying is to bring the car home and change all of the fluids. It is likely that they haven't been changed (except oil) and once you re-fill you can be certain that the fluids are correct.
As for the shifting, if the flex disc is cracked it will continue to get worse. Replacing it is simple and the part is less than $100. You should also try changing out the tranny fluid to Red Line MTL.
As for the noises in the rear...it could be several things, but the rear shock mounts are suspect and should be checked and replaced if needed (Rouge).
The seat sliding is the bushing, a very cheap part but a pain to replace. I wouldn't worry about it being something else, that issue is common.
I'm not sure that a "fuel injector service" is needed, you can get the same thing with some fuel additive and the brake flushing is very easy to do yourself.
Honestly you can save a ton of money doing these things yourself, most with simple hand tools. I would urge you to purchase a Bentley manual on the car and attempt to tackle some of these items. If you are unsure of a procedure, check the forum for a DIY, ask the question, or even ask if there is a member in your area that can help. Remember to check the regional forums as well.
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