As I am not planning to buy any place to live in the next few years, my situation is by far different from yours.... Therefore, it is hard to say anything.
Here, what you mentioned is tempting, but is also somehow risky, especially as we don't have much statistics on a stable HBD on the long term. Doing this for a fraction of the full amount that you are planning to inject in the flat could probably satisfy both the gambling feeling and security.
Anyways, I should probably stop mumbling here and leave you with your choices :)
Cheers!