Speedy,
Are you crazy? Don't you know how slow 123's are off the line? Just kidding, I have actually spent the last year resurrecting my fiance's that set covered in a barn far too long. The w123 is one of the last Mercedes that was built to run forever if it was taken care of with regular service. Once they are in proper tune (most sadly are not) you can expect a blistering 12-14 second 0-60 time and mid to upper 20's mpg's. I average 26-28 with a good mix of interstate/town driving.
I would rate the reliability as equal to a Porsche of the same age, again it really depends on how well it has been taken care of. You have to realize they are 25+ years old and stuff happens--I had a rather large transmission leak come up last week so the car is sitting for now. Once that is fixed I wouldn't hesitate to drive my 82 across the country but I have done a lot of work on it over the last year. Most likely any 123 you find will need a suspension refresh ($1200-2000 depending on how much you do), new motor mounts (under $100 if you do it), vavle adjustment (a three year old could do it), new fuel lines and filters, and lots of vacuum line connectors/parts--can get very expensive very quickly, and lastly all new brake components if they are original. I have tackled most everything myself and have just over $3000 in parts in my 82 right now. Part prices are not nearly as bad a Porsche and you can get anything you need as Mercedes still supports older cars.
I was amazed at how well my 82 drove after redoing the suspension and putting good Michelin tires on it--it just floats down the interstate. Cornering isn't bad considering how much it weighs and it will oversteer if you really push it. With a functioning turbo you will keep up with traffic but I wouldn't recommend any drag racing as you will loose. I have really lucked out with mine as it was taken good care of until it was parked, I even have the Euro delivery paperwork as a family friend purchased it new. You will need to really look for rust and will probably see a lot of junk. Good cars are out there but they tend to be much more expensive. I would think you will be looking at $5-7k for a dependable driver that is in great shape. Ours is more of a garage queen now as I don't have to drive it when the weather is bad. Here is a picture after getting pulled out of the barn.
Here it is all cleaned up.
