Larger aperture=larger opening for light to get in=better light gathering in dark scenes=High $$$
You're not likely to find anything in your budget that will do what you want really, IMHO.
Honestly, you're asking for a lot and probably don't even realize it. Low light performance is currently the holy grail in camcorders. In other words, you can spend just about as much as you'd like trying to get one to meet your performance needs.
I'm not trying to judge you so please take this in the spirit in which it is intended. You're probably asking for something that will work in any situation (dark rides, coasters, etc.) and while it is generally "do-able", you might just be disappointed in your results regardless what you spend based on your comment about aperture.
This isn't one of those problems you can just throw money at and make it go away. Low light is tough on any camcorder, even the "high-dollar" ones and getting satisfactory results in low-light situations requires time, patience and many times; manual override of the automatic settings of the camcorder (assuming it even has manual controls that allow you to do so AND that you have familiarized yourself with using them PROPERLY which require a lot of practice as much as anything else).
I'm not trying to discourage you but rather to enlighten you that bad lighting conditions are a challenge to any photographer and to any equipment.
I have all kinds of cameras and camcorders. I'm somewhat of a collector. Always in search of the holy-grail.
For video, when at WDW, I generally carry a Kodak Zi8 in my pocket. I recently purchased a Kodak Playsport and will probably carry that one next month and let my wife carry the Zi8. Is it perfect? No. What it is, is convenient, light, easy to stow and works reasonably well in most situations. 1080p HD, and in the case of the PlaySport, waterproof to 10 feet too. $139 on sale at BestBuy this week (beginnning tomorrow, I think).
I have enough stress trying to keep up with one of my my Canon Rebels and all its accessories at WDW. It's nice to just throw the Kodak in my pocket knowing it will do 95% of what I want it to do and it's in my pocket and at the ready all the time.
You may want to take a look at it. You may find it might just be exactly what you really want.
