Great, and good to know.
I can understand the eagerness to want to hit the ground running, and you will likely spend a ton of time processing cells and learning as you pass that time. If I can give you one piece of advice is dont rush the process, grow it organically and at your own comfort level. Dont expect you will get it right the first time either, especially if you dont make the investment in both time and equipment.
I say this more from the standpoint of safety and long term performance. It is important to note that stuff is dangerous, and doing it on a larger scale only compounds the risk. Poor process and procedure in testing is going to lead to quality problems, downtime, additional time and maintenance cost. To be honest these things actually need to be looked after.
This is already a respectable undertaking even with brand new batteries. To make matters more challenging, we are building with recycled cells of unknown quality.
I highly recommend building a proof of concept first. Could be a test bench setup, could be a portable battery pack, or even an RC car to get you more familiar with the general charging/balancing/discharging/storage-maintenance practices.
I love RC equipment so much in this hobby because its literally identical in practice, and my past experience with it REALLY help set the foundational groundwork for these types of projects. I highly recommend you get a good one as it is the Swiss army knife of battery work; it will become such a valuable tool (pack maintenance, capacity testing, charging, balancing, discharging, internal resistance testing, and a lot more) Its a great tool to validate the results from your bulk testing gear.
There are so many ways to accomplish this that you might change your ideas/design depending on what you learn. I feel like my project never really finish, they just evolve.
Either way, hope that wasnt too much all in one go. Go crack into some packs and start collecting cells, but dont be in a rush to actually slap it all together and get it into production.
Also, you mentioned you already have solar? Is this a professionally installed grid tie system? The charger/inverter combo, panel voltage, and other things are all going to be important in system design and equipment selection. And dont forget to research BMS system like Batrium or many of the other DIY systems people have created. There really is no set standard to how this is done
For a proof of concept type build, I made a little battery board as a BMS testing bench, but it could also serve as a learning tool. The 2nd part ill actually be using it to test BMS boards.
Here is some of the chargers and gear I like. Obviously find whatever works for your budget. Plenty of options from $20 and up.
You are going to have plenty of "B Grade" or "C Grade" cells that might not make it into your powerwall. You can always make a smaller project and practice soldering, spot welding, PCB, or whatever route you decide to go before jumping off the deep end.
Don't forget that EV cells are for the most parta larger 18650's. If you can gain access to those, then its another very valid way of building. I have some EV car cells that I plan to use myself. This is where the RC chargers really shine as they have better power handling and can better process "modules"
Hope that wasn't too much all in one go. Enjoy, have fun, and stay safe.