You have to keep in mind as time goes on the S9 will become unprofitable sooner than later.
I'm only a very small farm miner. An only my opinion here.

With the overall Hashrate at all time highs near 170 Eh/s and new units being cranked out as fast as possible by multiple manufacturers, (only currently slowed by chip an logistics issues right now), Next gen units on the Horizon (s19 XP), and Quite a few large scale Farms up and coming, in all, will eventually send that Hashrate way higher, and sooner than most think in my opinion, and render those S9's unprofitable fast. Total guess, and depends on Bitcoin Price action of course, but Id give the S9 another year of profitability.

Remember Crypto Winter, when the next one happens (and inevitably some sort of one will occur at some point for at least a while I would bet)
Just needs the right combination of Bitcoin Hashrate (difficulty), Bitcoin price drop, and S9's wont be worth running, and selling wont bring much return. I remember when Bitmain was selling S9's new direct for under $100 (before shipping cost) during crypto Winter.

A few S9's for a newbie getting their feet wet may be ok, A lot of learning in setting up a starter farm. But if your going into for the long haul and long profit. Go with the most efficient (new or used) reliable unit(s) you can.

I started in 2017 with 1 S9. Had 13 various machines at one point in 2018.
I have a mix a models (ages) from L3+ to S19 Pro running now (7 units total now)

Buy, run, profit,  HODL, upgrade, repeat.

Good Luck