I really advise anyone who's going to buy the aquatraders stuff to do some legwork and research this company before you slap down your money. There are reviews on reefcentral.com plus other places. I would be 'ok' with their pc setups (tho' expect no custmer servie) but would not touch the mh with a bargepole as I don't want my house to catch fire.
As far as lighting these tanks goes, I would bear in mind that 5 times volume just does not work in such small spaces. It is not even a very good rule for anything. Either the light intensity is enough , or it isn't. So you might need the same lighting for a 10 as for a 20, as frankly the dimension in any direction (except depth, distance form the bulb) is NOT going to make much of a difference to the overall light output versus the bulb strengh. Having said that the shallow water depth can really work in your favour here - I ran a 20 as a frag tank with about 50 watts of T5 with gullwings, and had crazy growth in SPS just as I kept the water 8 inches deep.
If you have a deep tank,then you will need stronger lighting than for a shallow tank. You should be able to work out how much more by knowing that light intensity decreases as the square of depth.
Another way to keep your lighting wattage down is to have really clear water, which effectively means carbon 100% of the time. That's also a good idea anyway as it will help remove some of the alleopathic compounds, as well as general gelvin (organic water yellowing compounds in german). If you don't know what alleopathy is, I advise you to google it.