Need first time home buyer/investor advice

So I have found a ready to move in house with a mother in law suite in the back. I am currently renting so I'm trying to make something happen. The house is listed at 149,000, I want to offer 140. I have an interest rate of 5.125 and I plan to put 20% down which comes out to 28k. I would like to use the BRRRR method and have calculated my monthly expenses to be around 900-950. When the time comes to rent I think I could rent the main house for 1250 and the mother in law suite for 650 which comes out to 1900..take away the 900 for expenses, I'm left with 1000/mo=12k/year. Should take me a little under 3 years to make back what I put in...but apparently my calculated COC ROI it's like 36.9% 👀 am I making any sense? Someone help..I'm so lost