As I expected, your amplifier has a nice amount of power. It is rated at 80 x 4 at 2 ohms... Almost all car speakers are approx. 4 ohms. So your amp will produce around 40 - 50 watts RMS at 4 ohms.
First off, I would not power the stock speakers with this. They weren't built to handle the power that your amplifier puts out. Also, after 10+ years, the paper cones are most likely wearing out and getting brittle. This just further increases the chances of blowing the speakers. I would highly suggest installing after market speakers. I commented on a few below.
The Rainbow speakers are rated at 100 watts PEAK... Unfortunately they don't show the RMS. This is common with lower quality speakers companies.This is just an observation... I cannot comment on the quality of Rainbow, however, because I have personal experience with them. My guess would be 25-30 watts RMS for those. They are definitely an option.
I know that Alpine makes a 5.25" coaxial that would definitely work well. Their Type S 5.25's are 35 watts RMS, 170 PEAK, and realistically should be able to take 45 watts without any trouble. These would be easy to install in the rear deck. For the dashboard, you might have to downsize to Alpine's 4" speakers. I haven't tried to fit a 5.25" in there yet. I can guarantee you that you would never blow these speakers. They can handle a lot. And for the price, they sound great.
Both of these speakers have their own appealing characteristics. The rainbows will basically be a drop in replacement. They were created specifically to replace the w124 speakers. The Alpine's will require a bit of modification to fit (rear deck very easy, dash I can't comment on), but you will get the legendary quality and durability that Alpine is known for. Either way, I think you will be happy, it just depends on what YOU want.
I really hope this helps. If you have any questions, just let me know.
Out of curiosity, what sub are you planning on putting in?
P.S. - I will be installing a full Alpine system in a friends w124 within the next week or two. I will then be able to comment on the fitting of the dash speakers.