Personally I would stay away from the m24, but I had a bad experience once. if you are looking for under 100 I would go with this
http://www.airsplat.com/Items/AR-WELL-MB06.htm The well mb06, I have seen a lot of these upgraded and shoot very well. They are type 96 internals with the exception to the trigger box, and the magazine is different, but barrel hop up and cylindar and internals are type 96 compatible.
If you do spend more $ to upgrade I would recommend eventually replacing the trigger if you want to go up to 550 fps, but no $ 100 bolt will be able to take 550 for very long. Cheapest one that I know of is the well mb05. I was pushing 595 fps on the stock trigger for 2.5K rounds until I replaced the box with an upgrade, and after inspecting the origional it was still fine with very little wear.