If you're QTing corals, I don't see a need for a protein skimmer unless you're feeding them. Even then, it's so easy to just do a quick 80% water change on a small tank to lower nutrient levels that I'd still skip the skimmer. Most corals can probably get by for at least a week with relatively dim lighting in a QT if you don't want to get MH for it.
The simplest QT is just a small tank, heater and light. Though if you're QTing fish, things get more complicated due to ammonia. In the past I left my QT up and running at all times, seeded with a small amount of live rock to create a population of amphipods, copepods and mysis shrimp for new fish to snack on while in QT. Obviously hyposalinity would kill those, but I can easily reseed the rock from the display once the fish has been through QT.