Most of my 1920's flips bring $100k in profit, but only after a year long slog to bring it to modern standards, while preserving the old character.They're not much more difficult than a 1950's house. 

Wealth preservation requires different strategy from growth. 

Investors contemplating investment in a multifamily real estate syndication deal should keep in mind that, while they will receive tax-advantaged income during the life of the investment, it may not be possible to defer taxes on capital gains after the property is sold via a 1031 Exchange.However, you can use a Deferred Sales Trust to help them gain tax deferral, liquidity, diversification and freedom to buy qualified real estate at any time tax deferred with your funds so you can create and preserve more wealth.

My interest would be to preserve them and to make such properties livable and up to code, but mostly affordable as well.

If the building has a heritage designation, or you simply wish to preserve the elaborate external trim or dentil or internal mouldings, these things are time consuming and skill sets to restore them, uncommon. 

From my general research on zoning concepts, it appears that rezoning requires opening up the dialog of what the city has planned or what they want to preserve in your area.

Capital preservation less than 5% returns.

These are ideal for investors who already have the net worth they need, and are looking to: reduce management hassles, preserve their equity, and not pay taxes on an exchange.

For an older home, there's another factor to consider: historic preservation.

I tried to make it feel more current, but also preserve some of the 50's character of this house.