I would not recommend a Bausparvertrag if you are planning to buy soon. They are basically a good deal for very young or poor families in order to (believe they can) buy a house much later in live... For example, if you save 40 bucks each month, your employer might fund a part of this. Or if your total annual income is below something like 20 or 30k per person, there are some direct payments from the government into this Bausparvertrag. But otherwise, the initial fees will eat up probably most of the potential gain from securing a better interest rate. Looking at 18 months, I don't think that there will be much of a change in the world economics...
I got my Bausparvertrag when I was 19 or 20, got some social benefits for it when I was young and poor(er), but never used it, even when I bought my house (the deal is now that I still get 3,5% interest on my savings and they will return the initial fee - much better than the below 3% mortgage interest rate that I got when I bought my house)