The easiest way is to get some sort of container ( large tub, 20 gallon tank etc) usually one of those are cheap. There are fish traps you can buy, I know personally how hard it is to catch a fish (I had to take down my 55 gallon after first starting up to catch a clownfish) but its worth it. All you need then is a heater, a cheap filter and thats about it ( take filter out if usung meds). I know 10-20 gal
tank kits are 20-$40 roughly. I used a 10 gallon for mine.
Ich is nasty. It will probably kill your fish if you cant get it out to treat it, if it gets bad enough. If you have inverts or if you are going to keep inverts in your tank I wouldnt suggest treating the main tank. You're going to have to have the fish in QT for about 7-8 weeks and leave your main tank without fish for that long. Its much easier to treat a QT than the main tank anyhow.
You can do hypo salinity or copper for ich treatment. Theres articles on both.