Often it can depend on your location, capability and financial situation that determines how you go about the project. My neighbour just bought a completed HG van and enjoys driving it for what it is. On the other hand a friend has spent a few years completing his HG Monaro and doing nearly everything himself. Both get the same amount of enjoyment.
As Sandaro advises it is highly recommended to photograph and identify everything carefully. Digital photography makes it a lot easier these days to store the photos for later use.
In Melbourne "Blast Off" is run by an enthusiast and can advise your options as can similar companies nearer to you. There was one company in Albury that tanked the vehicle but they closed recently.

Some have been caught out by taking their car to a panel shop and accepting they will work on it when time permits (to help keep the costs down). This can lead to lost or damaged parts and a long time before the vehicle is finished. Others enjoy doing the majority themselves and this relies on your capability.

While there are some who don't like Rare Spares at least you can look at the website for your car and the products available. This can enable you to get an indication of the potential total cost of the parts you may need and what is available. Then it is up to you if you buy NOS (New old Stock), secondhand or Reproduction. It is important to get some idea of what the project may cost you so you can determine how you are going to complete it. They also have some fitment videos to show you how door seals, glass run channel etc, are installed as a general indication.

At the end of the day you will have a lot of information that will allow you to choose how you go about keeping the dream alive.