It will be the concentric slave cyl.
Best to take it to a garage that know what they are talking about for the repair.
Depending on mileage may be a plan to swop out the fly and clutch and also fit an upgraded CSC.
Varta D47 Blue is the way to go:
http://www.ebay.co.uk/itm/Varta-D47-Battery-Nissan-350Z-Roadster-Z33-3-5-10-03-/272049447394?hash=item3f576921e2:g:--0AAOxy0bRTDbye
As what has already been said, however these seals should be replaced when a new screen is fitted.
Prehaps time for a trip back to the windscreen specialist
You require to do proper research.
All you are doing on here is asking questions and when you get an answer, that answer triggers yet another question.
Unfortunately on forums, you are as much likely to get bad advice as you are good.
In answer to your last question. if you want to remain NA and not have a decrease in power then keep the OEM air boxes and fit good filters as dense air is what you are after.
If you install cone shaped filters you will be creating the opposite of what you are trying to achieve. as you will have a decrease in power.
Stick with the OEM boxes and decent air filters.
Might be a plan to go away for a few days and do it bit of reading as research is key.
Yet to see a rear stat fail, however first time for everything.
Try running without the front stat.
I have also seen the impeller braking off the water pump.
If you are running the old style Mishimoto fan surround then it is possible to wire up wrongly which will result in your fans rotating in the wrong direction.
Having said all that, probably an air lock.
There really is no gradual way to get to 600 bhp.
Absolutely pointless spending a few thousand pounds to get an extra 20 bhp with an NA set up.
So guess that twin turbo is the only way with a built engine so with supporting mods 25k should sort you out.
I always find that the clips are rusted and seized on to the door shell and unless you are working from inside the door with the drop glass removed I have found that they damage to easily on removal.
You will have to send me a "how to" Jon: