Gotta agree with tubbed. I had some Alpines in my S10 Blazer and they put out enough punch with an amp. If you were going to do the speakers, you gotta have the amp too or it's just not worth the time and money.
All together, 6x9 Alpines are running about $120/pair and a 4 channel Alpine amp is about $200-300, depending on model. The speakers don't need all that much wattage, so any four channel will do. To add a sub, you'd need another amp, the woofer, a box, wiring, etc...plan on waiting if you're on a strict budget.
As long as you can do some installation yourself, wiring it all up shouldn't be too bad. There are a few guys on ebay that sell amp wiring kits for a decent price.
If it matters, I've had the same Alpine setup for five years in two different vehicles. I've run the systems pretty hard and I've never had an issue, so I'm an Alpine loyalist now
