Hi Gesine and Joerg,
Congratulations on taking the step to becoming boat owners! Ericsons are lovely boats and this forum is an excellent resource with alot of knowledgeable and helpful owners. The E30+ is a great boat, but you have certainly taken on a project in acquiring this one!
If you are totally new to the boating world, and don't know boats, it may be a good idea to have someone who knows boats take a good detailed look at it before you decide on how to proceed. This may help you to determine your priorities for repair correctly, and the order, so you don't do some work and then have to undo it to get at something behind/below there. Also to ensure that before you launch her (as you suggested in your post) that she is seaworthy enough to withstand that. Having a diesel mechanic look at the engine may help to determine whether it will start and maybe he can tell you if it is salvageable or needs to be rebuilt or replaced. Also, whether the fluids (oil, coolant, tranny fluid) need to be changed before you try to start it (likely a good idea), or if this is a waste of time and money if it likely won't start anyway.
Regarding your question about the wood bulkheads, others have replaced these successfully and there is good information and pictures of this work on the site. Ensure you familiarize yourselves with the correct way to epoxy, do some practice first, etc.
Regarding the naugahide headliner, it's hard to tell from the pics if it can be salvaged through cleaning. I once used a restoration company to repair a problem in ours, and they were able to do so leaving the repair totally invisible, so you might want to consult someone like that first. If not, some people have replaced the headliner with wood or similar material which looked awesome (again you can find the posts if you search for them).
I hope that you can both enjoy the process of the restoration and not get discouraged--others have done it, so don't give up, but do develop a good plan and do the work well to increase the likelihood of a good outcome.
Keep asking questions and someone here will likely be able to help.
Good luck!
Frank