500mw is plenty of power for just about anything you would ever want to do, however, you are correct in assuming location is critical. Location will not give you better range, that is a function of the antenna. What location does is eliminate or reduce the chance of noise screwing up the rest of your system. The more power the greater the chance of this happening.
On my x/a Hexa I have my 200mw tx mounted below the load rails on a solid cf plate, keeping it as far away as I can from everything else. The cf plate ensures noise from the tx does not emit upwards, where I don't want it.
Others, and I like this idea a lot, with rigid landing gear have taken to mounting their tx on the landing gear itself.
As far as the antenna goes, this is the key to the kingdom. Your standard dipole antenna has very limited range. You are correct to mount it pointing down, however, remember one thing, if you intend to skim the ground deflection from the ground can become an issue. If you fly meduim or high up, the frame itself can become between you and the antenna blocking the signal.
As for a recommendation for a better antenna, type in Bluebeam cloverleaf antenna into your search engine. They are an excellent antenna designed to give you good range and are not directional, so you will be fine close up and far out. Beware, there are a lot of people making these now and selling them cheap, ( Bluebeam is not expensive ), but they are not 'balanced'. You will probably also need an adapter to connect the antenna to your tx and rx.
You'll want the pair. One has 3 wire on it that goes on the tx and the other 4 wires that goes on the rx. The pole is flexable, so it can be easily bent and comes in very handy in a crash.
I personally used this set up on my 200mw tx and have taken my Hexa out 1/3rd mile without a single drop out or noise. I have no idea where the signal will falter.
With 500mw you will have very long range.
For even more range you could just order a single cloverleaf for the tx and a crosshair for the reciever. Crosshair is a fairly new design just now hitting the market.
Just keep in mind you really want to induce signal noise before you loose flight control signal. So plan accordingly. It is entirely possible to out-fly your radio signal, never a good thing. People who want to fly out a long way usually mod their radio to uhf to keep the signal.
Depending on what your radio is signal loss will vary. Futaba will reach out a lot farther than Spectrum, for example.
One other thing you want to do is it is best to have the tx on its own power source. This battery should outlast your flight pack. You don't want to be way out there and have your battery die beyond line of sight.
I use an 850mah battery to power mine and it is enough to last an entire day of flying without recharging.
