I've looked on the internet at both Top Dawg & Ikelite housings and read something about a mirror being used to see the lcd screen with the Ikelite. Do images seem 'in reverse' when using this method and if so is it something you get used to easily?
I've never used one. I know with some cameras it was possible to reverse the image on the LCD so it rights itself again in the image on the mirror. But you would need to check that out. I guess it wouldn't take too long to get used to framing in reverse but I've personally never done it.
Also Nick mentioned a 'blue-water' filter. Is this used as well as manual white balancing or instead of?
This is used in conjunction with manual white balance. It assists the white balance and also richens the colours up both with and without manual white balance. Without the filter you can't manually white balance as deep (it goes out of range and just refuses). If you have a rig that does not allow manual white balance then the colour correction filter helps the colour balance anyway. But if it's flippable then you would want to remove it at depth (say 30m+) where all it's really doing is cutting out light and increasing visual noise caused by the gain that the camera uses to compensate.
If you can afford it, get a housing that allows manual white balance. It's probably the biggest single feature that can improve the quality of your footage. If you can't afford a rig that you can manually white balance I have read (I think it was Drew on this forum) that Canon does better auto white balance underwater than Sony. I must admit my Sony's have not been very good at auto-white balance underwater. I haven't personally dived with a Canon.
As well as here you might want to check out the Digital Diver and possibly Scubaboard video forums where I believe there may be more owners of the lower end housings, especially Ikelite and USVH.