I would not class my skimmer as diy as it was not build by me and was custom build . really cant see any diy person building it, cost $3000 without labour, could have purchased a brand name skimmer for less.
Diy is ok if you are skilled in acrylic as it is not easy to glue tubing. And if you are new to the hobby and have not used various skimmers, lights etc then it is hard to really understand how the equipment should function.
Water quality is so important in keeping fish/coral I cant see the point of doing half a job, it is better to get things spot on and save money by not losing fish/coral .
FISH/coral come from nutrient poor water so if water quality is not excellant then fish will get stressed and diseases will appear.
I think big skimmers are important as it give you a much better margin of error in terms of getting you water quality up.
ANY skimmer will skim, however a undersized skimmer will leave to much disolved organics in the water which will cause problems due to high organic load.
Great book to get is THE MODERN REEF AQUARIUM BY SVEN/FOSSA