Get the slim if you have no alternatives, you won't be disappointed. It's slimmer, more battery life, lcd screen is good (there are oled fanboys but if you don't have fat to compare it will not bother you)
2 cfw ps3s used online from the beginning of "cfw era". No banhammer yet, maybe lucky/stupid/invincible. Just play it safe, don't do stupid shit online and you will be fine (or not).
According to my knowledge there are solderless devices that can do the work
I paid a guy like 30$ to downgrade my ps3 so i'm not really sure it's worth the trouble doing it yourself