You really only need the last one. If there are questions as to the accuracy of the last one, having some of the recent history may be useful. If you intend to have multiple configurations, e.g., with and without the back seat installed, then you need a W&B for each case or at least for the case covering the current configuration.
If you have a W&B sheet from before that modification and know what was deleted and installed, a new W&B can be computed from the last known "good" one. If that isn't possible, you can always weigh the airplane and start anew. Many people don't like to do this because all of the weight from accumulated dirt, grease, repairs, unremoved wiring, etc., etc., suddenly shows up.
I did this a while back and started with the equipment list in the POH, which had the original list from the factory delivery, and just deleting stuff that was no longer there and adding stuff that had replaced the removed equipment or known additions. It was a lot easier than I had expected it to be.
I don't know anything about Aero Space reports, but you can get the officially recorded documents directly from the FAA. The old process was to send in $10 and they'd send you a CD with everything on it, but now you can create an account on a new site and download the documents directly. Basically if it's not in the files they have, it doesn't exist.
Go to https://cares.faa.gov/home and create an account (which was a little tricky for me for some reason). Once the account is created you'll be able to directly research all kinds of documents, including all of the files (337s, etc.), for any airplane.
Usually manufacturers allow direct access of that sort of documentation, e.g., you can just look up AFM supplements for any installed equipment and download it. You can then print it out and add it to the POH. A more modern method is to keep all of the related documents, POH, AFM supplements, etc., etc., as electronic files in your EFB, on your phone, or at least on a thumb drive that remains in the airplane or readily accessible when demanded during a ramp check or annual or whatever. Given the large AFM supplements that come with a lof of equipment these days, printing them all out to add to the physical POH gets pretty ridiculous. The bottom line, though, is that you do need to have all of the relevant supplements in the aircraft, but they can be electronic.
You only need the version delivered with the airplane, i.e., specific to your serial number. There is a document, I think a spreadsheet or something, floating around that shows every POH version related to all the models and serial numbers.
I replaced mine with one from Staples.
It's good that you're trying to do things as correctly as possible, and this place is a good resource to get info regarding that.