You should start first with bolt on front and mid chassis under chassis stiffener braces like the ones shown in my avatar.
You only need to search PriusChat under "chassis brace" to find out. It been discussed "ad museum" here over and over, again.
Over the past two years, I did extensive research on braces, sold a number to PriusChat members who found them very effective. Now, I've been deemed a commercial entity and am not allowed to solicit sales here. Here's an earlier thread. A NEW version of my Gen III or Prius"v" box beam under chassis stiffener brace | PriusChat
It's more like a hobby. I just like making constant improvements and helping others at the same time. I'm retired and have a lot of time on my hands. I don't want a lot of business. That's too much like work.
I like Toyota stock/sanctioned upgrades so went with the Plus springs, rear sway bar and 17" wheels. Very happy with the improvements.
Suspension mods in most bang for the buck order (for Gen III): 1. new performance tires - better tires always means a loss of MPG (probably over $500) 2. TRD rear bar - reduces understeer and body lean (around $250 via Amazon) 3. TRD or aftermarket lowering springs - but you might have problems with ground clearance (might be over $400 installed) 4. aftermarket front brace - firms up the steering (about $100 via Amazon) 5. aftermarket mid brace - Toyota used a better brace starting in 2011 so you might not need this brace (about $100 via Ebay)
Great anwers, I really like the combo package for $500 xpcman. Luckly I still know how to work/install car stuff, so no need for labor. unless its a alignment or tire/balance issue.