Say you have a budget of £250.00 (MAX £300) for a GP saddle for a 6yo Sec D no expertise in particular (eg, some jumping, some dressage, lots of schooling)
If it's a new saddle you want then it would really have to be a synthetic however for around £300 you would be able to pick up a used Bates GP. The Bates saddle is adjustable with replacement gullets so it should fit your horse no matter how his shape may change. The new gullets only cost £14.95p from most saddlers. Should you ever decide to sell the saddle they hold their value better than a synthetic & you would lose very little if anything on a sale.