The first thing you need to do is choose a watchmaker who is known for respecting and working on vintage pieces, and who will work with you to decide what you want done. The full scope of work should be discussed before you hand over the watch to someone.
The key is that you need to decide what you do and do not want replaced before you start this process. You say you want it to be as original as possible, but also want it to be as water resistant as possible. While this may seem very specific to you, it isn't to me.
A watch is either water resistant, or it isn't. There is no middle ground really when it comes to something like a Speedmaster, since they don't have a high water resistance to begin with. You need to decide if you value water resistance over originality, because it is unlikely you will get both. I can't stress enough though that even if you replace all the parts involved, so pushers, crown, case tube, case back gasket, and crystal, that the watch still might not have any water resistance. If the sealing surfaces are all pitted, then it won't seal by changing these parts.
With regards to the aesthetics, that is an easier thing to handle most of the time. Most of the time hands can be used again (sometimes there are issues, but those are rare, at least on vintage watches) and the crystal is a simple replacement. Just ask to have the dial and hands used again, and to have the crystal swapped.
Also, I never refinish a case, bracelet, etc. unless I'm asked to, but many watchmakers feel this is an essential part of a standard service. So always make sure you specify that you don't want any refinishing done.
Hope this helps.
Cheers, Al
