QT's aren't that expensive. I've always put them off...but I just got my first QT for my new 120g FOWLR. No big fishies planned for in the DT, all small "friendlies" evenetually.
Anyways, got a 10g QT for about $60 with everything. I plan to use this for one by one QT'ing of new fish. Now, if I have to pull everyone out to treat, I'll get a bigger QT...even then, it isn't that expensive.
More or less, if you loose two fish to ich, you've probably lost more in fish money than what a 10g QT would cost to setup (again, for me, $60).
My QT equipment:
10g tank
Small Aquaclear HOB filter
submersible heater,
glass top
cheap-o light
PVC piping for them to hide in
=$60
Fish lives saved b/c of a QT=priceless
Anyways, with regards to your particular scenario. I'd go get a QT tank to handle all your fish, pull water out of your QT to fill and start treating. You risk loosing all you LR if you need to treat aggressively with medication in you DT.
Good Luck!