Regardless of what anyone may think, I doubt you'll be called a 'fool' by anyone here. We're clearly all Land Rover lovers, and the group runs the whole range from completely helpless to very qualified.
I have been driving Land Rovers for over 35 years, and have some knowledge of most every model, and much experience with a few.
I have owned several 88s, and a few 109s. While I love the traditional style and ruggedness of the originals, I have been driving a Defender 110 for the past several years.
If staying in the fold of Land Rover drivers is the first decision out of the way, the next thing to decide is what will give you the joy, with the most amount of practicality. As much as I enjoyed my 109s, I would have to say that it shouldn't be considered as a suitable replacement as a family daily ride.
The one you're looking at is still an antique, irrespective of the best restoration possible. It does not embody any of the technology, safety and comfort of even your '97 D90. While it's only my opinion here, I don't think it would be a good swap from a financial, dare I say, investment point, either. I hate thinking of any car as an investment, unless it's a '29 Packard or others seen at Pebble Beach.
A consideration is a fold down, forward facing (child seat MUST be fwd facing) single or pair of rear seats. If you want to stay with the Defender feel, style and pride, and your D90 is as good as you say, you might consider shopping for a Defender 110. It will drive more smoothly than your 90, have tons more room, look like the 109 to satisfy the style drive, and far more maintainable. A D90 SW is, in my mind, much more desirable than a soft top, as a year round practical car, but as the family grows, may be an expensive luxury.
If this doesn't make sence, consider a Disco. You still enjoy the pride of ownership, reasonable reliability with a new(er) one, and frankly, you ought to be able to take your 90 into the dealer and leave with a new '04
FWIW, I've even considered selling my 110 for a '97 SW D90.