And if political power can be bought through lobbying or corruption, there does not exist a democracy.

I have to disagree there, in that I think it's a bit more subtle.

There will always be people who seek power for self-enrichment and at the same time those people who see having power over others as a great responsability (who would probably be the best in terms of fair and honest yielding of that power), often avoid it exactly because they feel the "weight on their shoulders" would the too much to bare.

So you'll always have at least some people holding power who use it for personal upside maximization, including via corruption.

Your really can't have a perfect Democracy totally free of crooks in power, as even if you magically made it so, lots of people seek power for personal upside maximization and sooner of later some would get through.

Instead, what Democracy has is whole concept of the 3 independent Pillars Of Democracy, the Political, the Judicial and the Press, which watch each other and have some for of power over each other (the Press indirectly via influencing voters), and that's what's meant to create a sort of "dynamic" balance as crooks seek power but at the same time crooks in power are getting caught and thrown out (even punished).

Now, if you look at some of the most flawed of Democracies (personally I don't think they're trully democratic because their voting systems are mathematically heavilly rigged to create a power duopoly) - the US and the UK - you will notice that the Press was subverted first (and this has been going on long enough and deep enough that some people genuinelly believe partisanship - i.e. taking sides in Politics, so submission to a Political Party - in the Press is a good thing) and then the Political system became more and more corrupt, with in the US the additional problem that even the Judiciary pillar has been subverted at several levels by the Political pillar (not that in the US there was ever much independence of between them to begin with as lots of top positions in the Judiciary are of political nomination).

Anyways, all this to say that we'll always have pressures making political power "buyable", hence why its so important to understand the function of and protect the other Pillars of Democracy and their independence as they're part of the mechanics which pushes the other way, and whilst the system cannot achieve and remain perfect in a static way, it can achieve a dynamic balance that as the crooks get found out, kicked out and their deeds undone.