Welcome to the forum!
Mike's very thorough post covered all the highlights (and then some). It's very thorough; hope it doesn't scare you off

It's almost like having your very own surveyor's report.
Take heart: most of the needed or recommended repairs on a DS1 are pretty manageable affairs. That's the beauty of these boats, and why people manage to keep them sailing for 60 years or more in some cases.
You can probably sail your boat in between making some of the upgrades, but some should be attended to sooner than later. Others you may defer to the end of the season.
Because your boat has one thwart left in place, the CB trunk isn't totally unsupported; that's good, because otherwise you risk cracking along the place where the CB trunk meets the bottom of the hull; nevertheless, the missing one seriously compromises the strength of the overvall structure by letting the hull flex by widening and narrowing. While duplicating the mahogany seat would be ideal, a piece of marine plywood would do to get the structural support (and give you time to get the esthetics right later).
The best way to attach a bracket on the CB trunk would be epoxy glue (after sanding and degreasing both CB and the bracket). Not the hardware store "syringe" kind, but something like T-88 or GelMagic from SystemThree (or mix your own using West System). Glues are usually formulated to be non-sagging, which is what you want for the vertical application.
My DS1 has some cracks on the vertical sides of the cuddy opening (but none along the top). These cracks have not changed in many years. If those are the only ones and you can step on the cuddy deck without it flexing unduly, you may postpone that repair. I would, however, unscrew all the fittings and seal all the holes with unthickened epoxy (e.g. laminating epoxy like SystemThree's SilverTip, or, if you are in a colder climate, get some SystemThree ColdCure which will be handy for working during the shoulder seasons). (All epoxies need to be meticulously measured and mixed - getting that right is half the battle).
Sealing these holes is important to avoid the rebar hidden in the top of the cuddy from rusting. Once you get rust in there, the repair gets much more time consuming.
Getting the old foam out is definitely recommended, but you don't have to postpone sailing. When you select inspection ports for this, get the absolute largest that will fit. Makes it so much easier getting the old stuff out. Pool noodles are great - however, I used the "blue" styrofoam insulation instead. It's less prone to being waterlogged than the old white stuff, and now that I have the inspection ports, I can monitor these areas. In my case it was the wrong season to get pool noodles locally and I wanted to be on the water sooner than I could get them shipped to me.
All running rigging needs to be replaced occasionally: rope gets stiff over time. Look for my post
"Rope for various lines" in the Rigging section of the forum to get some ideas; you may over time want to follow the examples I give, or decide that some other kinds of rope works best for you. Mike uses dacron (polyester) for his halyards, I've switched to Amsteel which stretches less and is comparable in that way to the old wire halyards. Both have their advantages.
Standing rigging (wire stays) should be replaced when you see any hint of rust on it, or if you think it's original to your boat. If it fails, you will bend or lose your mast. Check out the tangs that connect to it on the mast (including all rivets) and the chainplates on the side of the hull. These are essential items, and you should be confident that there are no issues - generally these can be OK even for a boat your age, but a critical look is better than blissful ignorance.
Rudder blade: I would not buy a new stock rudder blade. The design isn't all that efficient and if you spend that money, might as well get something better. Or
make your own.
Have fun and enjoy your DS!