I'm wondering what coaches to use with my Marklin 39418 E41 374 in its two tone livery.

Marklin list the loco as looking like it was around 1966, and as Era III. I don't know how long it kept this livery.

Reading through the information that comes in it booklet it seems these locos were used for a wide range of duties, from main line express to main line goods, and suburban commuter traffic.

So some thoughts on what to use with it.

I already have the 43867 coach set with the 43868 entertainment car, which are listed as Era IV around 1985, which really seems a bit late, but otherwise may do for mainline use. I also have a extra 42259 Pike Car set that I could use which while listed as Era III around 1958, would still have possibly been around less than 10 years later.

For commuter use some Silberling such as 43897/8/9 which are listed as Era IV around 1975. This could complement last years second Insider 41275 model, and allow two similar trains (one electric, one steam) to have a shuttle service. But another possibility would appear to be the cream/blue 43165/43125/43335 set of centre entry cars that are due out shortly. These are listed as Era IV again, around 1984, so getting rather late again.

Are there any other coach sets anyone would expect to run around the 1966 time frame?

Yes , I'd agree with Ray .
This was the period where the DB was experimenting with "more attractive" color schemes for their rolling stock.
The culmination of that effort was the somewhat short lived "Pop" color scheme on passenger trains, which was introduced in 1969 and only really lasted untils 1974
(For example the first Mârklin Z gauge passenger coaches introduced in 1972 were in pop color schemes...)

From memories from these years , when I was regularly spending time in Germany for work, I remember seeing lots of E41s mostly in S-Bahn services .
The lok was equipped with MU control equipment and therefore were perfectly suited for push-pull duties!
It was used in so many ways, I'd say, you can imagine almost anything
In a book I have here, there is a picture of an E41 pulling a TEE train, simply because the E03 planned for the service had broken down , and there was nothing else available!

Enjoy!