A holiday home is an incredibly expensive luxury that became mainstream in the madness of the post Celtic Tiger property bubble. Previously it would be more associated with the moneyed classes than the mildly comfortable middle classes.
I'm assuming from your savings and the mortgage sum mentioned that you intend to spend EUR 150,000 on the build. I'll also assume for the sake of keeping things simple that you don't go over the budget as is common with self-builds.
It's possible that over the course of twenty years that between the EUR 25,000 capital employed and the interest on the EUR 125,000 mortgage that the combined opportunity cost and mortgage cost will equal the EUR 150,000 budget. That is before you consider paying off the mortgage principle. There are also other considerations such as utilities and property taxes.
Effectively you are possibly locking yourself into paying over EUR 7,500 per annum on holiday accommodation over the next twenty years. It would be wise to examine what EUR 7,500 per annum would buy you in hotel / self-catering accommodation over the next twenty years.
Your alternative is to invest your 25,000 in capital elsewhere and continue to save and invest. You will have the option of choosing where you holiday over the next 20 years instead of being locked into this one location. You will have the option of not holidaying in a given year, and potentially saving yourself EUR 7,500 on accommodation alone, if you are overstretched financially.
You could argue that you will get rental revenue from the holiday property. Then it becomes a investment/holiday property and aside from potentially overpaying for your holiday accommodation you are looking at an investment that may not be performing.
It's understandable to overpay for a primary residence for emotional reasons but can you really afford to overpay for your holiday accommodation for emotional reasons?