According to price, here's my ranking....
85 300TD
85 300CD
85 300D
81-84 300TD
82-85 300CD
82-84 300D
80-83 240D manual
For non turbo 300s, similar ranking, followed by the 240D auto bringing up the bottom.
85s are most desirable, due to rear end ratio and torque converter change.
Next are the turbo diesels from 81-84.
The oddball is the manual tranny 240D, demand exceeds supply.
Non turbo 300 diesels from 80-82 are next.
Anything 79 and down is much less desirable than the last of the breed. Glow plug system, upholstery, lots of little differences. Most of these early cars have now gone to MB heaven, so specific early stuff is getting hard to find.
280E and 280CE are in the hunt, but the US versions had rather low power compared to the euros, and mileage was poor for what power you got.
Euro versions have pitfalls, some can be hard to insure, and getting unique parts can be a hassle. Upside is lower purchase price, cool bumpers, and the possibility of option combinations not available on US versions. Figure euro versions are worth 1/3 to 1/2 less than US cars. Of sourse if you find a 5 speed manual 280CE, this rule may not hold.
What to look for:
Condition - If you see a little rust, you'll eventually find a hell of a lot more before you are finished. Rust free is THE big thing on W123 chassis. Big premium for no rust. Southwest cars are good for this, but most hot, dry country cars require substantial rubber replacement, and this can get to be very expensive if done right.
Maintenance - Records, records, records. So what if the car "shows" low mileage. Odometers are notorious for breaking. It's not hard at all to "clock" the odometer when you are fixing the broken ones. Maybe it has low miles, maybe not. But if you have a paper trail showing consistent increases, it's much easier to prove it.
As a subset of this, an original car with no record of major suspension work, subframe/diff mounts, seals, etc, etc. is likely going to need thousands of dollars poured into it to bring it up to speed. A well maintained car with high mileage, but with all repairs and maintenance kept up to date, will only need to be enjoyed.
Appearance - It's not hard to spend $4k for a nice paint job. Yea, yea, I know you can put it on with a roller and wet sand it for $50.00. What YOUR tme worth? Top quality paint materials alone will cost near $1k. So, how nice is the paint and bodywork? Nice cars get top dollar.
Interior - Carpet sets and seat covers and dashboards are not cheap. I laugh at the adds that say perfect interior, except for driver's seat, driver's carpet, and dash has cracks.... Ever try to find a driver's side carpet that is perfect? Got to buy the whole set $$$$$. Passenger seat can be subbed for the driver's if you swap the armrest, but still you have to find a perfect 20 year old seat. This takes lots of time, what's your worth? New uhpolsterycan be had, but again it adds up. Dashboard, priced one lately? Sure, you can install a coverlay, but I can tell it's not original, can't you?
Driveline is important, but it can all be refurbished if needed. A well kept car with records should not have any surprises in this area, unless something catastrophic has happened. A nice, well maintained car that just lost it's tranny CAN be a really good buy! A beat looking car, on the other hand, is likely to have deferred maintenance on the mechanics as well.
Good, clean 300D turbo sedans have been consistently selling on ebay for $7-10K. Some low mileage, several with well over 200k on them. Let's assume that represents the value of a ready to go, not needing anything significant car. Start deducting everything the subject car needs from that price range, and you then know what to pay for the car you are considering. Doing this, often will result in a negative value. The only way around this is to introduce your labor as a substitute for the mechanic's, and don't pay your self anything.
Two reasons to do this: First, I enjoy working on cars, it's my therapy and hobby. Second, I can get a car that I otherwise could not afford, if I can recognize what it needs and am willing to do it myself.
Jim