I'm too poor to buy a house.
I'm not rich enough to buy a house.
They both mean the same if you are talking not having the financial resources to buy a house.
But there are different if your listener want to interpret your financial well-being from these sentences.
The first one emphasised on your poverty ( TOO poor, meaning you are poor all along), while the second just emphasised a lack of money now, not that you are poor or anything like that.
So, the 2nd is best used when you can't afford anything. Poor is a negative word