We have hot water using coils in the oil furnace and no external tank. You can run one shower as long as you want and two for normal shower duration. We never run out of hot water.
Replacing the coils may be the least expensive near term fix if you are thinking of solar down the road.
For a separate external unit 40 gal should be fine for one or two people but a 60 gal unit may not cost a lot more as installation charges should be about the same for 40 or 60. The 60 gal unit may be better on any future home resale.
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