Welcome to TFF.
We're starting from scratch with an empty tank - We need to buy gravel for the base, but I prefer finer gravel, closer to sand, although i've been told this suffocates live plants roots and that I should avoid it - Any basis to this argument?
No there is not. I have play sand in all my tanks. Plants will grow very well in it, and all fish will like it (some like cories need it). As a former fine gravel substrate aquarist, I wish I had made the change to sand much sooner than I did. I've had it for 6+ years now. Play sand is inexpensive, realistic in appearance, completely safe. The dark grey is better, but whatever, avoid any white sand as this will affect fish.
From research it appears the Eheim filter would be a wise choice over something like Fluval or smaller brands, for ease of use, although i'm sure that will open a can of worms on here!
Eheim has the proven track record like no other brand. But before deciding on filters, you need to settle on the intended aquascape/fish, as fish can be affected by filter flow. A 240 liter (66 gallon roughly) is large enough that you have options such as active fish requiring currents, or sedate fish needing none, and the filter is your tool here. Live plants also make a difference.
In terms of the set-up, how long should I look to leave the tank to settle before adding in any plants or fish life to it? I know we need to do appropriate tests to make sure chemical levels are adequate.
If you intend live plants, aquascape the tank with the plants on day one. Once the plants are showing signs of growth, and provided there are some faster-growing species (floating plants are ideal for this, plus their other benefits for fish) you can slowly add fish without worrying about cycling. A tank will not fully establish biologically for a few months, once the fish are also in it, but with live plants you should not have any issues.
Tests...the API Master Combo liquid test kit is a good investment. Ammonia, nitrite, nitrate and pH tests. All needed initially, and down the road periodic pH and nitrate tests; ammonia and nitrite at the first sign of a problem (like a dead fish).
How much basis do the equations on cm of fish per litre hold? We'll be adding fish periodically so I would assume its more of a personal feeling/not too crowded tank over a mathematical equation?
While this body mass/water volume ratio has some value, it is frankly useless. Each species of freshwater fish has evolved for a very specific environment (meaning, water parameters, natural habitat, numbers, other species, plants, water flow, etc). The species will be healthy in such conditions; any adverse conditions will likely add stress and that weakens the fish and makes life much more difficult for the fish.
Where I am going with this: a group of 20 cardinal tetras can be completely happy and healthy in a 20 gallon tank which provides a replica of their habitat; a group of 20 danios in the same tank will not be healthy or happy at all, because what they need is not being provided. Yet the water volume and fish mass ratio is basically identical.
Fish requirements is programmed into the species' DNA, so we must understand the requirements for each species and then provide the closest replication we can if we want healthy fish. This is why you need to sort out the intended aquascape with the likely fish species in the proper numbers (shoaling species must have a group or they will be stressed) with suitable tankmates first, then build the aquarium to provide the requirements.
