The situation seems quite familiar to me. I had, or rather have, two adjacent plots of land; on one stands a house built in 1953, and the other was freely buildable. There was quite a bit of capital, but of course not enough for a large house. Then came all the advice from people who neither know current construction technology nor can use Excel. Sorry, that's how it was.
In hindsight, I have to say: Unfortunately, I did not use the help of an architect back then – if I were to do it again, I would do exactly as 11ant recommended. But not everything can always be enforced within the building community (i.e., spouses), and I was very lucky to have two competent advisors who were accepted by both me and my wife. They first removed all the pub talk from us and explained how to build and what the difference between old and new buildings is.
Result: We exploited the buildable area down to the last centimeter, did not afford golden faucets but designed a house five years ago that is only now becoming mainstream. When I go into the house on the neighboring plot, even though I grew up there, I just want to tear it down because I simply do not have enough money to maintain it.
I honestly never understood the life comfort of a new build until I had a well-calculated and built new house. One where such small things as the air exchange rate simply add up correctly. The heating system does not have 87.2 pieced-together components but is a no-frills air-water heat pump with underfloor heating. We considered a fireplace but quickly dismissed it because of the one-time and running costs, environmental impact, regulatory uncertainties, and a quick calculation of heat input.
Renovate a house? Why are the bargains probably so cheap? Sure, depending on the year of construction there may be objects worth renovating, but they are usually not the bargains. If you tear out everything that needs renovating, nothing is left except a dreadful floor plan.
I would say you first need to agree with your husband regarding the loan. A newly built house lives for many decades; if you take out a loan for anything, then it should be for that. You two should agree on that.
Then the plot must be secured, in such a way that you can do whatever you want with it. Without any kinship clauses in the land register, just simply: your plot.
Then follow 11ant’s plan. Aim to move in two years from now – if it happens sooner, great.
And just reading the description “our two children are 2 1/2 and 5 years old. I work full-time, while my husband is a stay-at-home dad and househusband” then in my opinion even just the thought of completely renovating a very old house is simply absurd.
As a building team, you have to do your thing. For years (of course only when we were not around) people whispered that it was so pointless that we were building new at all, that we were building so big, and so on. Then came Corona and no one complained about the size anymore. Then energy became expensive and no one laughed at energy-efficient construction anymore. Then they visited in winter and no one laughed at the central ventilation system anymore. Then they visited in summer and no one laughed at the cooling anymore.
So especially when such remarks come up, you have to think: Who am I actually building for here? Who will live in it afterward? The pub brothers or me? That’s why my house is exactly as I want it.