Hi Riwls and welcome to the party.
Yes that’s correct you will get the good condition payment (GCP £250) when you return the car in a reasonably condition in 3 years time.
The other payment you will get now, £750 will be paid a few days after you collect your new Motability car and in your case will be a welcoming bonus whereas in we old hands we will get a loyalty bonus. These welcome and loyalty bonuses are a one off payment not to be repeated but the GCP is always paid when the car is return at end of lease in a reasonably condition allowing for fair wear and tear.