Sounds a little like the adventure I had with my Classic SE CDT a few years ago, and if you check my blog and go waayyyyyy back, you'll see some of the upgrades I performed on mine.
(Only car I've ever regretted selling, wish I'd kept it as a second car)
You can certainly add rear vents, you will need to remove the centre console, cut the blanking plate off the bottom of the aircon unit, and fit the tube and vents (I actually have some tubes going for free if you need one). Changing the whole system is dash out to replace the aircon unit, not for the faint hearted. As long as the aircon is working, I never saw the point in doing it.
Front fogs is doable, but, the very early CDT's are missing a wire from the fusebox so be warned - you can add it in though.
Plenty of scope - if you are in no rush just source the bits as cheap as you can and fit them at your leisure
