H! Here to introduce myself and get some basic help

[KhoalaX]

So my ign is KhoalaX. I just started a couple days ago, and have recently gotten the first 7 badges. Gio senpai is too strong rn so I have a few questions that could help me get stronger, and some other ones in general.

-what's the best place to get/fine high lvl pokemon(not looking to capture, just need a good spot for a lot of xp)

-where can I find a poliwrath(or anything in the evo chain?) I have 2 fire types rn, but no water, and since my blaziken is getting kinda sucky, I was hoping to get a fighting water type pokemon to replace him

-do the strangers do anything(I've found the purple one at veridian and the green one at seafoam)

-what is the weird star tower thingy in the seafoam cave?

-are any of the free pokemon good?

-Do the pokes up for sale in the "legal" pokemon store ever change? Are they good ones?