There has to be some device to control the current on any LED. This may be built into a package with the LED or it may be separate. So each LED is around 3 volt and a set current depending on the type, 320 mA is common, so with DC the limit is 75 volt before it needs protection so if a 0 ~ 60 volt power supply is used then around 20 LED's could be powered from the same power supply.
However often we use a two stage system with a power supply dropping the voltage from 230 Vac to 12 Vdc then a second device powering lots of three LED's with the correct current. This latter stage could be a complex PWM chip or a simple resistor, as a result packages of LED's can vary vastly on efficiency from around 40 lumen per watt to 100 lumen per watt.
Using LED's we really have two ways to use them, to light a room or decorate a room. Using the three colours on strips is great decoration but not really any good to light the room sometimes the lumen per watt on the decorative LED's can be down to 20 lumen per watt.
Although you can get LED's and drivers (in theroy a driver controls current) in extra low voltage packages, it becomes complex working out what you have and getting replacements, in general low voltage (230 Vac) are better as far easier to replace and upgrade one has not got to look for some special device which matches what you already have.
Larger LED units tend to have a better lumen per watt, As a 300mm square lamp or as a replacement for a fluorescent tube likely will be 100 lumen per watt, smaller spot lights more like 60 to 80 lumen per watt, however even at 60 lumen per watt that is far better than tungsten so more down to what looks nice than how many lumen per watt. For my caravan I can buy 50mm spot lamps rated 10 ~ 36 volt at 100 lumen per watt but each lamp is expensive I can get 3 house types lamps for the price of one caravan type so I am personally satisfied with 60 to 80 lumen per watt as they cost less to buy in the first place.
So in real terms the lumen is only to tell you how many lights are required and as long as you use all 230 volt types swapping for larger or smaller is easy. So to answer question personally I would not buy any transformers or drivers or power supplies I would buy lamps with them all built into the lamp. Up to around 1000 lumen per lamp the LED works well, as you exceed 1000 lumen per lamp the LED becomes very expensive. I will give my kitchen as an example, a 5 foot fluorescent tube with HF ballast costs around £35 with around 5400 lumen output, to get that with LED need two tubes at £18 each and that's not including the fitting to get 4800 lumen and when the fluorescent fails it costs £4 each for new tube when LED fails it costs £18 each for new tubes. So using large LED lamps does not work out.
However with 50mm spots the 8W cold cathode which is really a fluorescent tube costs more than the 8W LED and gives out half of the light. And the folded tube used in CFL are also short lived and no where near as good as the LED version. And to my mind the 5 foot fluorescent tube does not really look that good lighting a room even if it is cheap. Comparing 300mm surface mounted LED lamps to the old 2D fluorescent I think they are about equal to each other it is personal preference.
Other than special locations like maybe a bathroom I would not fit extra low voltage (12 volt) lights. Simply not worth the hassle and expensive.
