Without more specific words in the sentence, none of your three meanings can be universally implied, out of context. And, since you haven't given any context, that's what we must assume.
All of your sentences mean that you are generally thinking about these devices. There is no implication of picking a model or of purchasing one.
To imply version a, you need something like:
I'm considering which [kind/type of] car I should get that is good for driving to work.
I'm trying to decide which house cleaning robot to buy.
Using words like "considering" or "deciding" tells the reader/listener what you are doing. Emphasizing your decision making process by using "which" tells them that you have several options to choose from. Without, it's simply a yes/no situation, though it needs a slight word order change.
For version b, you need an verb in addition to "thinking".
I'm thinking about getting/buying a car for going to work.
I'm thinking of buying a robot to clean the house.
When you have decided to get something, using "thinking" is no longer appropriate.
I have decided to get/buy a car so that I may more easily get to work.
I've decided to buy a robot that will clean my house.
