What is the best sealer/waterproofer to put on a basement floor that I just want to paint. New construction, no water problems, but washer & drier & water heater are in basement and add to moisture in the air.
I'd suggest one of two options:
I much prefer the look of stained concrete to painted concrete, but that's obviously a personal choice. We stained our concrete basement floor when we finished it and sealed it with a off-the-shelf concrete sealer.
If it's new construction you shouldn't need to put down a waterproof layer before painting the concrete floor. Below the slab should be a waterproofing system that prevents moisture coming up from below.
If you wish to double check that rising moisture is going to be a problem, take a look at this answer here on SE.
Below are a couple of questions here on SE that you might also wish to have a read through (as I believe they contain some useful and relevant information to your situation):