DAW / PC / Mac -- you could take a look at Reaper
, works on both systems, is inexpensive but very deep. Lots of routing options. Can record both midi and audio on the same track. Very good automation. And even comes with some useful plugins. I've been using it for years on a PC, and occasionally on Mac. It is very stable and does everything I need. FL and Reason do things their own way and are perhaps not as easy to move to from regular DAWs like Cubase or Logic. I don't think Macs are any easier to work with than a PC. It is more to do with what OS you are used to.
The tweaking and modulating you speak of has a lot to do with plugins and synths. Make sure you go for plugins that offer a lot of hands on tweakability and easy midi control of knobs/sliders. If money is a problem keep an eye out for deals ... there are always deals (sign up for newsletters), like on Waves plugins each week , or Melda Production plugins half price each week. BTW, those Melda effects are not the best looking things but they are really very good and with a lot of details and modulations you don't at first notice. Worth checking out.
I've used a lot of midi controllers, but now I primarily just use an M-Audio Axiom keyboard:http://www.m-audio.ca/products/view/axiom-61#.Vksvh3arSM8
Plenty of sliders and knobs to use, plus controls for your DAW (pause play etc) and not a bad feel on the keyboard. And not expensive.
There are of course lots of controller keyboards to choose from. Novation is another brand, they also offer automapping of controllers to various software.
It's never easy to say what is the best for anyone, depends on personal taste, feeling comfortable with the system, and finances. But I think the issue of creating ambient is more a matter of the right synths, effects, and technique. With the right skills you can create great ambient with a few field recordings and a dash of drone. You need to demo stuff and find what sounds right to you. Many good synths these days already come loaded with effects and great modulation and tweaking abilities. I'd say concentrate a lot on what synths give you what you need.