Weil McLain boiler rebuild questions

salio

I have a Weil McLain cgi5 series 1 boiler that's 20 years old and it started leaking a couple years ago. As a long-time finish carpenter and back yard mechanic on a tight budget (cancer will do that to you), I pulled the heat exchanger apart myself, re-sealed it best I could and re-assembled everything. It worked fine for a while but now I'm just going to replace the entire heat exchanger with a new one from WM.
In the process of rebuilding, I replaced several other parts such as installing a larger expansion tank, and replacing the direct vent inducer fan, the burner tubes (which are now stainless steel), and the ignition module.

Questions:

1) Are there other moderately priced parts you would keep on hand in case they go bad in the middle of a cold spell? I'm thinking I'll order another ignition module since I've gone through more than one of those, but how well do the gas valves hold up, or the transformer, or that electrical box with all the wires in it, or anything else?

2) Also, when I pulled the heat exchanger apart last time it had a bunch of hard, small, light colored chunks of something that had accumulated there - probably 3 or 4 cups worth of this stuff. It's well water that has a fair amount of iron in it. I've read that the entire system ought to be flushed. It's a converted gravity system with about 530 sq. ft. of radiation, and the original pipes are maybe 4" pipes, so a lot of water. How do I go about flushing this thing and getting it really cleaned out?

3) Also, I regularly get air in my radiators throughout the heating season. Is that because of the leaking heat exchanger? It seems to me that I've always had to bleed the system a few times throughout the season even when the boiler was newer, is that normal? How do I figure out where the air is coming in, and do I need an air scoop installed? I do have the little, cylindrical air bleeder but do I need something better? Yeah I know, "post pictures" - not happening, or, "get a professional in to look at it" - probably not happening either for now at least. Should I run a boiler sealant through the system before hooking up the boiler to maybe seal up some small leaks? and how do I get that oxygen free water in the system? maybe it was a leak all along that kept calling for more water, and ended up rotting out the heat exchanger prematurely?

Thanks ahead of time for your input.