Honestly, the spacers should be the least of your worries. You can usually buy MDF spacers on eBay if you don't want to make them, and they take moments to install. Changing the wiring on the door speakers is a much bigger pain. The stock speakers wire up the front HU output only to the speaker set, so you might want to redo that, but then you need a crossover (included in a component set) or decent amp... it gets a bit muddy.
A 'crossover point' refers to the target frequency where you split sound out so that higher frequencies go to the tweeter and lower frequencies go to the woofer. Usually it would be something between 1kHz and 5kHz. At a certain point, sound is too low for the tweeter to handle well, and at a certain point, sound is too high for the midbass/woofer to handle well. You want to make it so that you have audio across the spectrum, but you want to turn the tweeter off before it gets bass and vice versa. And you want the two to transition naturally too. If say your tweeter starts to performs poorly on sounds below 2kHz but your woofer is good to around 4kHz then you might make your crossover point 3kHz. The same goes for the range between your sub and your midbass.
I think getting a very basic set of speakers plus a very basic Alpine/Nakamichi/similar single DIN headunit for something like $250 total would probably give the most bang for your buck, but I would also want a sub (plus its box, amp, and wiring) if I did that and you're at $500+ at that point.
If you want inexpensive decent quality speakers and are willing to learn a bit, another good option is to take a look at the woofers and tweeters at
www.madisound.com. Not just the 'car audio' stuff. The listed cost is generally for a single speaker, and you need 4, but you can get very 4x decent SQ speakers for about $40-60 plus shipping. You would also need crossovers or an amp/HU that can take care of that though.
If you like it you could pick up a Pioneer DEH-P880PRS or P800PRS HU for less than $200, check for the pico fuse issue, and that will cross over internal to the HU. Then get cheap speakers from madisound that have a nice flat frequency response curve, hit the auto-EQ and time alignment to adjust to the car acoustics, and you'll have very good sound. Then add a sub when you're ready. An Alpine HU plus the imprint kit will give you similar sound and processing but you don't get the crossover internal--you have to buy an amp or use a component set to get that, and then you need to bridge the channels or give up output power.