I agree with the previous recommendation that you may get additional value by buying a kit since you need so many things at once.
We're currently building a pond in our backyard that will be about 6000 gallons. It's in a sloped part of the yard so DH is working on a block retaining wall for the "low side" now. When that is complete, I'm considering buying a kit such as this one:
http://cgi.ebay.com/Mega-30x40-Savio-atlantic-Koi-pond-skimmer-kit-M304-/380241256710?pt=LH_DefaultDomain_0 This kit has the liner, underlayment, savio skimmer, submersible pump that I can use for now, waterfall weir/filter falls, tubing, etc. Ultimately, I'll probably add a pressurized bead/sand filter, so I may have to get a different type of pump than what is in the kit or what I already have purchased.
We really debated on the bottom drain and skimmer issue for some time. Since we will add koi, I believe they will really be necessary. I also have a massive crepe myrtle next to the pond (20+ foot diameter of branch spread), so there will be leaves, tiny annoying ones! I already have purchased two bottom drains and two external sequence pumps.