first...Mitchell would be the place to go for the wiring diagram...that's what I use when I run into issues at work while installing...unfortunately I do not have access to Mitchell myself, but use a friends when the need arises...so I can't get you the information needed, but know you will find it there...
Now for the issue of replacing the amp...the only concerns I would see with replacing a factory amplifier would be as follows...
1. An aftermarket amp would want to see 2 - 8 ohms (in most cases, some may not be able to go down to 2 ohms)...you may end up having to replace speakers...
2. Depending on the output of the factory headunit, you may not get enough signal to the amplifier to get the proper signal out to the speakers...
3. you would still have to run the main power and ground for the amp as it would not be recommended to run it of of the factory power wiring...
4. Placement of the amplifier...may not work in terms of sizing and or air movement...you may have to find a new place to mount the amplifier and extend any wires...
Why do you really want to keep the factory head unit? You mentioned you wanted to keep costs down and that you wanted to maintain the steering wheel controls...is that the only reasons...
Personally I would replace the headunit and not worry about replacing the amplifier...instead I would just figure out the pin outs of the amplifier and rewire the harness so that the wires coming from the head unit to the amplifier were connected directly to the wires that run from the amplifier to the speakers...this would totally eliminate the need for an amplifier...
Decent decks can be found for under $150 and some come with wireless remotes that could take the place of your steering wheel controls...and pricing versus amplifiers...for somethign decent...you would be ahead of the game going with the deck...
The only concern I would have with a setup such as the one I described would be going back to the speaker resistance...aftermarket decks want between 4 and 8 ohms...go much outside of this and you are asking for trouble...
Just my 2 cents, but the wiring diagrams you would need can be found through Mitchell...
Jeff