As mentioned earlier, it will be difficult to do any meets without a car. Car hire in Tokyo is expensive and very few UK/US based car hire companies have outlets there. You really need some contacts there. Be careful when signing a car rental agreement. You need to check it is in English (unless you understand Japanese Script) so you can understand what you are signing up too.
However, you can get to Nismo (HQ) quite easily by train (a 15 minute walk from the station) but I have to say that it is somewhat of a letdown when you get there. The little shop there is not always open and you need to check opening times for the shop and museum (Ground Floor of Office Block). There is a man behind a PC who does a very good impression of the Travel Agent in Little Britain.............I kid you not. You can look over into the small basement workshop as the shop has a window in it.
In Ginza there is a showroom which is really easy to get to. You cannot miss it and is only a block away from the Sony Building. It shows off Nissan's latest wares. Occasionally they get their race vehicles in. There is a shop upstairs which sells T-Shirts, caps, model cars etc.
Have a great time. It is a great city!