I've built these for a marine tank of larger size as well as the sort
of size you describe..

Great! Someone who had already done this!

Its been on on these projects "in the pipeline" for a while

Marine ply is very expensive because it is free of voids as well as
being water resistant. For this application, WBP (water and boil
proof) is adequate, readily available and a lot less expensive.

Not worked with ply much before, and appreciate the tip! Trust this is
avaliable from my regular timber merchant.

For this size, I would say that 5mm would be just about OK as long as
you are not going to house the ballasts for the lighting in it. If
you are, then I would go up to 9mm.

Intention was to run the lighting as an Arcadia T-bar
(http://www.aquatics-online.co.uk/big...adia_I_bar.htm) with the
lighting fitted on "runners" on the tank from so that it can move forward
and backward to gain access to the tank, without having to remove the
lighting unit. In this setup there is no additional weight on the lid.

I avoid using anything metal near a marine aquarium, but if it's fresh
water then less critical. Even so, I wouldn't use aluminium. For
this application, I would construct using entirely wood including a
wooden frame. Glue together with an exterior grade of woodworking
glue or even epoxy.

Use plastic hinges for the lid or avoid hinges altogether by having
the top drop into place on the frame. It's just as effective.

Great Tip? Plastic hinges may be hard to source, but two panels work work
ok, and as you suggest be easier.

For finishing, I use glass fibre cloth and epoxy on the inside. You
can get this from model aircraft suppliers. It doesn't need to be
heavy grade as the main purpose is to protect against water.

With fresh water, you could use several coats of a solvent based yacht
varnish and get almost as good an effect. Either way, it needs to
cover the inside and the surface in contact with the tank itself.

It is a freshwater tank, so I will consider both options.

For the outside, it's less critical and almost anything will do
although should be splashproof at least in case you drip any water on
it.

Final color will be black, so I am sure something can be found to acheive
this.