I would get as close a size and style match to existing, as I could, and then plan on painting the entire house, with the proper type paint for aluminum siding. There is no way to get a good cohesive look, since your existing is very old already, anything else will look mismatched.

Can we see a picture of the whole house?

I will try to send one now.

humble cottage. looks better when crape myrtles are blooming!

With an addition there are two approaches: make it appear seamless integrated so it looks like it was built as part of the original home or celebrate it as different. Given the design of your addition, I'd try to make it a seamless integration. To do that, I'd remove the aluminum siding from the left side of your home and use the salvaged aluminum siding for the addition. I'd replace the siding on the entire left side with other siding and try to match the exposure of the existing siding.

I like additions that look like they have always been there so I would do as close as possible and paint the whole house

White cedar shingles is what we do on Cape Cod. Mix the weathered shingles with clapboard.

If you can't get white cedar shingles, I think Hardie Board has a dupe.

You can stain the shingles as well.