Depends on the pleco. Usually a pleco will feel better on a substrate that looks like them. A yellow and black pleco, like L-333 or L-134, will feel better on black. Whitish plecos will feel more comfortable on a lighter substate, and some it doesn't matter. For your tank, I wouldn't get a pleco that grows larger than 6", with leaves most of the peckoltia genus and the hypanancistrus genus. Many beautiful plecos in those two groups.
A sponge filter will definatly NOT be enough. I'd suggest a nice HOB filter, like Penguin or Emporer, and a canister filter. I love Eheim's, but I have heard many great things about Fluval canisters as well.