Hi Mark and welcome to Webforumz!

Ok, there's quite a bit there, so i'll try to get through it!

1. Colour scheme, fonts, etc can all be controlled by a stylesheet and that will cover your entire website. Design is, in my opinion, a very different topic. It's best handled with a program like Dreamweaver, with graphics made in a seperate program, such as Photoshop. However placement of items on the page, such as your navigation will need to be done seperately. This could be accomplished by simply using an include line in any number of languages from ASP to PHP.

...talking of which...

2. PHP and ASP can be used to do pretty much exactly the same things, however the languages are quite different. ASP is developed by Microsoft and as such, only runs on windows (it can run on Unix, but it's a fairly poor emulation for more advanced programming). ASP is usually scripted in VB - So I expect you would feel very much at home using it. You can install IIS on windows 2000, XP, etc, very easily and run ASP scripts locally.
PHP is a different beast. It's open-source, like Linux and mainly runs on Linux. It uses the C and C++ style of language, so if you'd prefer that you might want to give it a go. You can get PHP to run on windows, but it's nowhere near as fast and refined. PHP has a lot of packages available for it and has many SQL functions built in.
Each has it's own strengths and weaknesses and both will scale pretty well until you get to huge systems and then both become redundant. It really comes down to your preference or the situation.
One thing here though... to get ASP to work with a database on a large and frequently used site, you will not want to use access as your database - You'll want to use MSSQL. This can be quite expensive. PHP uses MySQL regardless and is far more robust to make this transition (and is much cheaper than MSSQL). However, ASP, using an Access database is normall cheaper than PHP with a MySQL database!

3. The best way to manage lots of content is usually a database with a content management frontend! There are free ones available out there for both ASP and PHP...

A good site to look is : http://www.hotscripts.com/