A few things to consider:
1. inheritance doesn't mean quick sale... We are deep into it, the house we are buying was put up for sale in September, previous buyer pulled out, we went sale agreed on it on February 6th and the EA assured us it would be a quick sale as the vendors were eager to sell. Still waiting on probate grant. So unless probate is already granted, it may take months.
2. planning application: you already have one on the house you are buying, even if it's a complicated build. While the one you like more may have issues with pipes, manholes, etc that render further building more difficult, if not impossible
3. Child with a disability: are you looking for a separate build where your child can live more or less independent, or would you be better off adapting your own house? I have a friend who initially planned for a granny flat for her daughter, but as the years passed, she realised that her daughter couldn't live without supervision so she'll eventually get her home modified
4. I assume the 100€ is a mistype, but if it's considerably cheaper, it may be a better option to clear your mortgage early as a child with disability will likely mean that he will be on a disability allowance, and you would be wise putting aside money for his future.