This can be done. The best setup that you could assemble would be to power all 4 of your 6.5s with your amplifier. The first two channels would be on the fronts, the second two channels would be on your rears. The 5th and 6th could be left alone for the time being, that is until you get your sub.
OR
You could bridge channels (1 with 2) and (3 with 4) to ultimately have 2 channels with high power leaving the 5th and 6th channels to operate normally. Then you could connect those 2 high power channels to your fronts giving you a more meaty sound stage and connect your 5th and 6th channels to your rear speakers. This setup would work until you got a sub. When you got your sub, you would just take the speaker cable connected to the amp and put them in their respective channels (non-bridged). That would leave your 5th and 6th channels open and able to be connected to your sub.
Now speakers.
For the <99$ range, I like the Polk dB 6.5s that would run you 99 shipped per pair.
http://www.sounddomain.com/sku/POLDB650I would also suggest the JBL GTO 626
http://www.sounddomain.com/sku/JBLGTO626 as these are both really great performing speakers. I have to run to class, but I'll be back online after lunch.