1. Get some pictures so you can negotiate a price reduction with the seller.
2. Figure out how you could drain the water away to a sump/catch basin outside the garage before it gets there. Then, put a pump in the catch basin. You will want to divert as much as you can before it gets there, perhaps with a small hump that will keep it away from the garage door.
3. Calculate how much water you will have to pump to keep it from getting to your garage/basement. 1" of runoff per hour from 1000 square feet is about 11 gallons per minute. Calculate your runoff area and put in a pump that will handle at least 1" of runoff per hour.
4. Remember that power outages often accompany heavy rain. If you need to pump to stay dry, you may want a generator to run the pump if power fails.