I think your best way to establish a fair price is to Google 'Hanoverian horse breeders'. Many come up. Pick two (or preferably more) that live near you and ask if they would mind looking at your horse and 'appraising' him or as least looking at photos, a video and/or a copy of his papers. Let them know up front that you are going to use their information to establish a fair price for him and will offer him for sale after you get a real feel for his honest value on today's market.
If you do this, you will not only get a feel for his value, you will probably get an offer or two from these breeders or from people referred to you.
Most commissions on private sales run 10% to 20% of selling price. It is usually at least 20% if the person also does the advertising and takes care of all correspondence and shows the horse to prospective buyers. It is usually closer to 10% if the owner keeps the horse and another person only refers prospective buyers to the seller.
It sounds more like you need someone to handle the horse and the showing of him and keep him fitted. If you get someone else to appraise him, you might also ask them what they would charge to fit, advertise and sell him.
As long as you get 2 or 3 different people to choose from, you are not going to taken for a ride on the deal.
Keep in mind that when you buy a horse, you are looking for a horse and the horse is going to cost full price (retail price if you will).
When you NEED to sell a horse, you are looking for buyers and you will receive 'wholesale price' at best. Be prepared to sell at a loss -- especially if you add up cost + care + training and then deduct selling costs and commissions. You are far better off taking less now because you will probably realize less the longer this drags out.
Hope this has been of some help. Good Luck