I have recently stumbled across Arduino and it seems I have missed the boat! I love tinkering and this is right up my alley.
Then you're going to have some tinkering to do for this project.
I was thinking about buying a basic arduino board to play around with when I remembered I had an old RC helicopter i've not used for years (http://www.xenonproject.com/rc-helicopters/electric/double-horse-9051a-eagle-remote-control-helicopter-p-36362.html)
That's a fairly small 'copter...
I was wondering if it is possible to use the arduino (along with other controllers) to control the helicopter in flight, I've seen videos of others doing this stuff and it is amazing, The helicopter is not one of those small ones that break in 2 flights but it's not amazingly great either (it was like £30.00) would this helicopter be big enough/have enough lift to carry the parts required? and what about the balance after adding stuff to it, I imagine it doesn't fly the same?
Whether it has enough capacity in order for you to add extra parts is difficult to say. Since none of that information is likely available anywhere (unless someone else has measured it), you'll probably have to measure it yourself. Basically, you'll want to set up something to measure the mass/weight it can lift. One way would be to add mass in some form (rice, maybe?) that it could lift by attaching it to the chopper below the skids. Keep adding mass until it can barely take off so you know the maximum capacity - then you'll have an idea (be sure when you measure that mass, you measure the mass of -everything- you added, not just the "rice").
Another way would be to implement a lightweight "balance beam" with a platform on one end to which the helicopter is attached, and the other end pressing down on a digital scale. It should be well balanced so that it doesn't affect the measurement in any way (easier said than done). Securely attach the helicopter, zero out the scale, and let 'er rip!
In fact, I would bet that you could find some kind of similar measuring device/devices in the R/C hobby market...
Where can I find more information on lift/balance?
You might want to look into various R/C hobby magazines and sites, especially those dedicated to helicopters. There have been quite a few books written on the subject as well. This forum may or may not be able to help you with more than a general sense (though maybe someone here has done some experimentation, or flies R/C helicopters for fun - you never know!). I can say, though, that no one will be able to tell you -exactly- how much -your- helicopter can lift unless they own the same one and have measured it.
Yes, adding weight will change its balance, depending on where you add it - the best thing to do is to keep it balanced by distributing any added weight equally under the helicopter, and also realizing that the "center" of thrust (and balance) is directly below the main centerline of the main rotor.
Also, your helicopter probably doesn't have the capability to lift a standard Arduino (it's not lightweight), but there are many smaller Arduino and Arduino clones out there that could possibly work (and the various sensors you'll need are fairly lightweight themselves), but realize that everything you add will require power, and you will already be robbing flight time just by adding weight, so you may need to add a larger battery, which means more weight, which means a larger battery...
It can be a vicious cycle, and not something that is easy to calculate (basically, your ultimate tradeoff will be flight-time - which is already pretty constrained).