While I hope that you are comfortable with any decision you make, I have to point out that you didn't give us a lot to go on. What I mean is, you have a 5 speed 300E and a web page, but the page doesn't show us the W124. How can we agree that you should dump your existing car without seeing it.
As well, you know that any younger W124 will need the same work as your car is facing. We are all facing the same repairs, just some of us have longer to budget.
In all seriousness, your potential car makes mine look pretty bad. While I have done a lot to mine, it still needs:
paint - $3000
fuel pump relay - $100
dash air control - $1000?
monovalve - $50
sportline conversion - $1300
wheels rechromed - $250
When added to the money I already have in the car, $8K, it looks like I need the reality check, not you!