Our boat was built late 2000. It is identical to the several I have seen from that period to 2004 excepting how it was set up with spinnaker or gennaker, and how mainsheet is handled. Options were extra tankage, bigger batteries, bigger alternator.
Because the boom comes back so far I really see no way to run a framed bimini. You might look at the photos on yachtworld of the red one in Seattle for $289K. Its been for sale for years, is the most heavily equiped one out there hence the silly price. They may have figured out the bimini.
The boat has minimal tankage, and if you add much you will screw up the performance. I have 75 gal fresh water, 27 gallons fuel and 11 gallon holding tank. The only obvious place for more storage is under v berth which screws up the balance of boat. All the D39 I have seen come with 100' of 5/16" chain for rode. You could cut off as much as you dare and replace with line to save some weight to offset the increased weight if you added extra fuel or water storage below v berth.
If you plan on cruising the stock alternator should be replaced with a high output and external regulator. We went with 100 amp Balmar and high quality v belt and are very happy. We also have the upgrade 2 8D battery house bank, AGMs. Most of the D39 from that era had Yanmar 3JH3E engine, putting out 38-40hp and a saildrive. Yanmars, if you don't know are high rpm engines. They like to be run at 2800-3200 rpm. Even well insulated alot of noise comes out of engine compartment. Something to consider if you expect to motor a lot.
With the aft sheeting of the main coming down to traveler right in front of bimini , mounting navigation instruments like chartplotter at the helm would be dicy. I have not seen anyone do it. Our chartplotter/radar are down below at Nav table and we use a hand held Garmin in the cockpit. If you mounted a panel of instruments at binnacle and hooked the mainsheet during a tack or jibe you would likely rip it right off. Boat comes equiped with Harken's self tending traveler, so when you tack it takes a field trip to other side. Very nice set up for performance sailing but makes hanging a bunch of instruments off binnacle area problematic.
All the boats I have seen or know of for US sale have been two cabin , aft head boats. I understand that in Europe the boat can be set up for charter with 3 cabins, 2 heads, a silly idea. One of the main attractions for crusing is how large the two cabins are plus a nice size head/shower and the huge garage which obviously would go in a 3 cabin layout.
Not having kids and doing most of our sailing as a couple we only looked at 2 cabin boats.
Another nice thing about Dehlers, at least from that time period, is the fit and finish and quality of most everything. The mattresses are very nice, the upholstery is first rate, The self storing hatch is a huge timesaver, two seconds it can be opened or closed. Curved steps on companion way so that when boat is heeled over you have a nice level step. All the covers to the bilge are edged with teflon bumpers so the covers don't rattle in a seaway. Is all this stuff worth an extra $100K on a new boat, no not close, but when you buy the boat used that premium goes way down obviously.
One of the treats to the boat is light air sailing, we sail in pretty congested waters, lots of boats. When its low wind and we have gennaker up , we have yet to be passed by another boat except for a couple J boats also flying colored sails.
good luck with your search