How to start with technic? I have bought myself the technic set 9393 Tractor. I really like the model and I want to dig deeper into the Lego technic universe. I am not focused in building pre-designed models (although I enjoy it, and it would be  a plus), but I want to have a decent set of technic parts, such that I can build models on my own. So what is the best way to get a basic set of bricks:


Buying some sets and take the parts from the sets? (rather buy a big set, or several small ones?)
Buying used parts as bulk?  
Are there non-model technic sets, similar to the traditional Lego, for which they have door sets, wheels sets etc. ?


I know this question might be a bit broad. So let me add that I am not necessarily want to build motorized models right away. Also, I do not plan to build huge models, more in the size of the 9393 model.
Here are the two things that I would recommend:


Get yourself a big set if you have the budget for it. These will have a large variety of pieces and once you've built the models from the instructions you'll have a good understanding of a lot of different building techniques.
Watch eBay or similar sites for large lots of Technic parts by the pound. These can be a little hit or miss, but the price per piece is far lower than most other ways of buying parts. Keep in mind that prices tend to be higher around the holidays.


There used to parts packs for Technic like what you are describing, but I believe they were removed now that pretty much everything is available from Pick-A-Brick. You can also get any parts that you want from bricklink.com, and if you don't mind used parts, this can be a pretty affordable option.