My goodness... just decide what you want instead of comparing interest rates!
With a sum of 50,000€, even a 2% interest rate still results in double-digit monthly costs. That really makes no difference at all. Even a consumer loan with 3.x% or so could be cheaper than the KfW loan, since no land charge registration is required. Do you have such a low income that you're afraid you can't afford all this?
Either you figure out how much you want to renovate, or you buy directly with a 50k consumer loan so you don’t have to be tied to a bank. You then pay it off when you want to do a total financing.