Whether to combust Oil or Gas
There is no perfect answer to the A4 vs diesel replacement quandary...
Some random thoughts:
If you buy a boat cheap(er) because it has a gas engine, you will sell it cheap(er) someday. No penalty.
Boats with a gas engine have a more limited market pool of willing buyers.
Time is money... when you lack time. Having to sit on a gas-engine sailboat for a long time before taking that lower offer will cost you in moorage, insurance, and lost $ value while your boat money is tied up.
Whether you think the buyers are well-informed or mis-informed, many of them indeed will not even look at a gas engine sailboat, and quite a few more will also not consider a really old diesel and/or a raw water-cooled one.
Pressure to "go diesel" could also come from your insurer and surveyor.
Two important factors are going to be the length of time in years that you plan to keep your boat, and the intrinsic worth of the boat itself. An Ericson is one brand that will continue to attract buyers wanting good design and construction quality.
Major Upgrade Diesel Engine $$ have to be viewed as a longer-term investment in your pleasure in using the boat. For most of our boats, burning a half gallon per hour really reduces time circling the fuel dock, too.
And someday when you decide to sell, you will quickly atrtract more potential buyers.
One note on the process of upgrades to any 30 year old sailboat: There will soon come a day when the original A4 will start to need of lot of spendy bits like a new fuel tank, controls, panel, and etc. With a new engine you start over with a new trans, panel, mounts, alternator, etc, etc -- and a warranty.
Strictly a Personal Opinion: Like some other posters here, I and my wife would never have purchased a gas inboard sailboat when we were shopping.
Fair winds,
Loren