Quicker and easier way to convert custom HTML templates into a Wordpress Theme?

I've been doing a lot of Wordpress site development in the last few years, and get especially excited whenever I get to create a custom Wordpress theme.  

Typically, I'm given either multi-layered Photoshop files or a flat JPEG images, and I then go about the task of converting them into a responsive HTML templates. When I'm done, I convert those templates to a custom Wordpress theme.  As I mentioned, I enjoy this very much -- but it has proven to be very challenging and time intensive at times, and I was wondering if there might be a more effective way for me to go about it.

One of the things I've always wondered about is if it would be more practical for me to simply modify or adapt an existing theme (ie: the built-in "Twentythirteen" theme) -- or if it would be less confusing for me to just start completely from scratch.  

This time around -- I am thinking of starting completely from scratch, .. using only my CSS (and to NOT re-use any of Wordpress specific CSS styles that are part of an existing Wordpress theme).  Is this a dumb idea?  What problems might I encounter if I choose to use this approach?  Should I re-think that idea, perhaps?  Please advise.

Also -- it seems that most of the "How to create a custom Wordpress theme from scratch" tutorials that I'm finding online are outdated, and don't really touch on any of the new HTML5 specific features, or updated techniques relating to custom theme development. Could anyone here steer me to some updated or more recent tutorials?  

Lastly - is there some tool or service that I can use that would allow me to quickly and easily convert my HTML templates into Wordpress theme files?  For example ... you type in a URL to your live HTML template, click on a button, and it spits out some PHP files? I know that this might sound like a ridiculous question, .. but it sounds feasible to me, so I thought I should ask. Does such a thing exist?  If so, where?

Thanks!
- Yvan