When i flash a new custom rom, does it get rid of other things that i've flashed before?
As of Lollipop all custom ROMs have transitioned to the new system.new.dat + system.transfer.list format, so the /system partition will be fully overwritten, which means bye bye GAPPS and Xposed. If you migrate to a different ROM, you'll also need to wipe /data (not including /data/media which is your internal storage).

Then again, flashing GAPPS/Xposed again isn't that hard, is it?

As for ROM recommendation, I think it's against SE's rule to do that here (XDA wouldn't allow that either), so better go try everything yourself. I myself am a die-hard CM fan.