Honestly, if you're got a hacksaw and can make measurements to a reasonable degree of accuracy, its so easy to do the plumbing you'll wonder what the fuss was about, I actually find it fun! I'm sure there are people in Christchurch who have done it before and would be glad to help in exchange for a free coffee.
With the return, I'd suggest drilling another hole in the bottom of the tank next to the overflow and running it up through there, along the length of the tank inside the hood, and have it coming out at the far end. Has the tank been built yet?
Rather than doing the perspex thing and cutting slits, I'd suggest using tinted glass for the overflow (its usually cheaper too) and getting some overflow comb from the LFS to silicone on. That way you can easily remove it for cleaning. I've never actually run anything on my overflows, and the only fish I've had go over were eels (fire and tyre track) and small tetras. In my 6' I siliconed a piece of glass in just above the water level so the gap is only about 10mm for the water, too small for any of my fish to go over, except for my tyre track who some how managed to squeeze through.