Flush vs Drain and Refill

Question.

I have a 1992 citroen ZX 1.4 carb. Prob. foreign to you Americans, but I’m an american living in the UK at the moment so this is the car that i’ve ended up with.

My heater isn’t working and so I’ve embarked on the coolant system diagnosis… I need to at the very least drain, flush, and refil w/ good antifreeze…

I need to know: I have an aluminum engine, thus need to be careful about chemical flushers right? So, would you recommend using a chemical flush agent on my car? Or just water to flush out…

I will refill w/ at least 50% antifreeze mix, though I might use the advise of the person above for 2:1 mix… make sense.

Do I need to worry about the sump of the system? I have never actually heard of this small detail. Where is this sump depostory of old crud? Is there any way i can actually access it and clean it out? If so (without buying a new water pump) i would be interested in doing it.

I am not sure how corroded the system is, but it hasn’t been flushed out int 6 years and w/ my heater not working (OK not sure on cause yet, I do hope it’s not the matrix) I need to make sure crud is OUT!!).

Thanks V much