To the one who quoted (second hand or less) from R’ Dovid Cohen, please ask him yourself. I believe that what you repeated is different from what he originally said. The integrity of the information decreases exponentially with each transmission.
I believe that the source for maaser kesafim is “Asser Ta’asser” in Parshas Re’ah. (Asser Ba’avur she’Tisasher)It is not deoraisa but the practice is learned out based on the possuk. I was taught to consider the chiyuv as a business partnership with Hashem. 10% of what you earn goes to your Partner. As such, expenses related to earning the money are deducted from the total earnings to be divided. I never heard of deducting other expenses such as rent/mortgage and insurance. If a Rov says so, go ahead, but find out if he really did say so.
As far as what ma’aser money can be spent on, I know that there are many, many different ways to pasken. Answering this question is pretty useless on this forum because of the variance between poskim. Ask yours. (for example, rob mentioned buying seforim. I have heard from different Rabbonim that you may, you may not, AND that you may but only if you buy it for a public bookshelf. So what value is there to tell you how MY Rov paskened?)