I don't know if you're willing to buy online or not but...
1 & 2) given your speaker budget of $150-$225, you could consider the following:
For around $190, Infinity Ref 6010cs comps in front, Infinity Ref 6012i coax in rear.
Also for around $190, Alpine SPS-171A comps in front, Alpine SPS-170A coax in rear.
For around $230, you could do CDT CL-61A comps in front with CL-6x coax in rear.
I suspect you could assemble a low-end Kicker or Pioneer setup for around $160-$170.
(It just occurred to me that I don't know what size speakers are in the 2000 4r. The above assumed 6.5s in the rear. If they're 4s or 5s, your rear coax will cost even less).
What you might also consider is spending ~$200 for the front comps and neglecting your rear speakers for now (they're just for fill anyway). That gives you more component possibilities including Alpine R-series, CDT CL62, Infinity Kappa, JL VR, etc.
3) Lots of 2-channel amps and several 4-channel amps in this price range. IIRC, cardomain.com currently has a sale on (presumably 2005 model) Orion amps.
An option you may not have considered would be to combine 3&4: That gives you $350 to work with. Get a 4-channel amp such as the Audiobahn 6004T (75Wx4@4ohm) which I've seen for $150 or less since Christmas. Use 2 channels to power your front components, run your rear speakers from the head-unit's built-in amp, use the amp's remaining 2 channels to power a sub. If you get a DVC sub with 2ohm coils, you could do 150W@2ohm per coil or bridge the amp channels and run the coils in series for 150W@4ohm (I don't think you can run that amp @2 ohms bridged). It's not a whole lot of sub power but unless you're looking to wake your neighbors, it's probably enough for now.
That leaves around $200 for your sub and enclosure. Lots of 10" subs can be had for under $100 (check Circuit City or Best Buy) but for sound quality, I'd consider a 10" Image Dynamics IDQ10D2v.2 at $150. That leaves $50 for your enclosure. For a sealed enclosure, the IDQ10 only needs around 0.3-0.5 cuft. That's tiny. You could build it for $15 sans carpet.
Shipping costs would probably eat up the remaining $35. Buy beer with whatever remains.
Online places you might consider include cardomain.com, woofersetc.com, crutchfield (especially their discounted scratch & dent stuff)...Note that I'm not affiliated with any of these.