Like most others, I agree that the speakers are the most important components for the sound. The main speakers matter the most, then the subwoofer, as it deals with much less of the audio spectrum, and whether it will be used at any particular moment will depend on the content recorded. For example, a solo guitar is not going to be working your subwoofer.
As for the speakers matching, I have gone to identical speakers in all positions (other than subwoofer obviously), and I will never go back to anything else. Some people don't care if a sound changes tonal character when moving from one channel to another, but I do.
With sources, if we are talking about digital ones, then the differences will tend to be minor between different brands and models of the same type (i.e., different BD players from different brands will be very similar in sound, different CD players from different brands will be very similar in sound, etc.). That will be especially true when using digital outputs (e.g., optical, coaxial, or HDMI). With analog sources (like turntables and various analog tape decks), though, they can differ dramatically from each other. And good ones tend to cost quite a lot.
As for the budget allocated for each (as per the opening post), there is no set amount to say, as different budgets will set different constraints. For example, if one is really on a tight budget, one ought not spend anything on surround speakers, a center speaker, or a subwoofer, and just get a decent pair of speakers and something to drive them. One pair of decent speakers would be better than a bunch of crappy ones. So one would need to know the total budget (and whether there will ever be more money in the future) to say how one should spend money. And also how good one requires. For me personally, if I had a total budget of $1000-1500, I would go with just a pair of speakers rather than trying to get a complete surround system at that price. Other people who care less about quality may well prefer to get a complete surround system, even though it would mean getting lesser speakers.
So it is a question of both finance and personal priorities.