I'm not saying nothing can blow because i don't know how good you are at wiring
but assuming you can wire it up correctly you will be fine
Yes there should be one fuse usually 30 amp by the battery on the positive wire to the amp, Take it out till you have it wired up. For a second hand 4 channel amp with atleast 600 watt capabilty will set you back about 50-100 i guess. I can't recommend an exact one but for good affordable make i'd look at jbl gto stuff maybe.
You could wire the 6x9's into the amp and then the sub which would sound better than wired to the head unit and then wire some back door card speakers to the head unit so you will have 6 speakers running same as me sounds mint
i also have a 4 tweater system going made by fli it sounds crystal