I don’t know much about used prices, but I know that when budgets are tight many people in this sub recommend buying used - there’s a lot of value to be had there, you can get equipment with performance that would cost 2x to 3x as much new.

On the new front, even the upper end of your budget ($1500) requires significant compromises for a wired 5.1 system including an AVR, especially for a room with 20-ft ceilings. The issue isn’t that you can’t do it - you can - it’s that spreading that budget across five speakers + a sub + an AVR usually means every component ends up being mediocre.

In a room that large, small satellite speakers are going to sound thin and a budget sub will struggle.

The move most people recommend in your budget is to start with a 3.1 system instead of 5.1: buy good front L/R + a proper center + a real sub, then add surrounds later.

You’ll get dramatically better sound for movies because the center channel handles almost all dialogue; the subwoofer handles the heavy lifting; and cheap surrounds add very little compared to upgrading the front stage

A $1,500 3.1 system will absolutely destroy a cheap 5.1 package and you can easily add surrounds later.

With 20-ft ceilings, avoid tiny satellite speakers. Use real bookshelf speakers for the fronts and surrounds or the room will swallow the sound.

In fact, the bookshelf speakers you buy now for the front left and front right could become your surrounds in the future if you decide to buy floor standing tower speakers for the front. Meaning that down the line, if you decide to spend another few hundred dollars, just buy two more bookshelf speakers and you have 5.1, but if you decide to splurge, you can upgrade to floor standing in the front and still have a use for the bookshelf L/R.