A free piano will still cost to move, tune and possibly repair. A dud won't be the bargain it seems and will cost you to remove. Bargains can be found but try to bring someone who knows what to look for.
We started with an entry level Yamaha weighted digital piano (£550). We didn't want to spent too much in case our children changed their mind about the piano. Has a volume button and plug for earphones but we never used them.
Three years later, we upgraded to an acoustic. We could feel the difference between the acoustic and digital piano. We debated a top end digital but went for an acoustic in the end. Piano teachers all advised the same as well. Once my kids have learnt, I'm sure we'll sell it and buy a digital again - when they'll play for enjoyment - but we feel it's better to learn on an acoustic.
We spent ages looking for a Yamaha U1. Over a year ago, prices for secondhand U1s had already risen by a grand due to Brexit weakening the pound. Most are imported from Japan. This is what I was told by the dealerships I visited. A secondhand U1 from the 70s was c.£3k - £3.5k back then.
Agree with the poster above about trying out lots of pianos. We spent almost a year looking for the one! Played many. In the end, bought a Kawai upright as we preferred the more mellow sound. Both our piano teachers also recommended Kawai over the Yamaha. Paid £3k secondhand for one less than 10 years old. It was a lot (for us) but we consider it an investment. Tuner costs us £65.
We kept the digital. It's upstairs and we use it a lot still so it wasn't money wasted. The difference between our admittedly not top end digital and acoustic is huge. The acoustic sounds so much better, pieces can be played with more nuance and my child now says there's less of a discernible difference to the acoustic used in the exams.
Depending where you are and if you can travel, we also looked at dealerships (secondhand) up North. They weren't any cheaper than down South but sometimes there was more choice. There's also the Piano Auction in Holborn. You can pick up a great bargain there. Friends of mine bought an amazing Steinway. I almost bought one. Had my piano teacher check it out. We loved it. My first time at an auction. It was nerve wracking! Unfortunately, another lady also loved the same piano and outbid me :(
Another thing to consider is what you want the piano for. If my kids and I already knew how to play or we were not bothered about exams, I'd have stuck with our entry level digital as it meets those needs perfectly. I have friends who are accomplished pianists. They are playing for enjoyment only and any old piano (if it's in tune), digital piano or keyboard will let you do that!