This is one of those things that's not particularly practical to DIY, though of course you CAN DIY it.
All LEDs (bare, DC) are "dimmable" - the trick with making an AC-connected LED-fixture dimmable is all in the power supply design. When you DIY that, you are biting off a considerable bit of advanced electronics, and you're probably also making yourself liable if the fixture causes a fire, since you presumably won't be spending huge gobs of cash to get it UL, CSA or whatever applies where you are Listed. So you need to make sure it does not, can not and will not be able to be blamed for a fire, or your insurance company will do exactly that and fail to pay.
For decent-quality LED lighting, most of the hard design is thermal design. Keeping the device in the safe operating region while providing adequate light out, and perhaps also trying to keep it somewhat safer than that to reduce the rate at which it "ages" and puts out less light over time. Optical design can be an additional consideration, depending what the light needs to do.
How many lumens and light type you need are not answerable questions - it depends on what the fixture is supposed to do.
You can (fairly easily) buy various bits of generally low-perfoming (sometimes cleverly made to look vaguely like better performing) LED lights from ebay and stick them together, but they will generally not perform anything like a well-built fixture. UL marks on the power supplies, if present, may or may not be counterfeit.
You can buy high quality high-power LEDs from reputable suppliers and custom-fabricate heat sinks and circuit boards (sometimes one and the same.) It's a non-trivial undertaking. Dive into something like this and see how you feel coming out the other side: http://www.cree.com/~/media/Files/Cree/LED-Components-and-Modules/XLamp/Data-and-Binning/ds-XPL.pdf