Used iPhone SIM card

I have an iPhone with Docomo, but it just broke. I wanna buy a used one from GEO. Is it as easy as buying it and popping in my SIM card?

https://ec.geo-online.co.jp/shop/g/g2336422662655/

https://www.sofmap.com/product_list.aspx?gid=UD47010000

I also went to Sofmap today.
The store clerk told me if I buy a used Docomo iPhone, I can just put my old Docomo iPhone SIM in with no problem!!!

...But the iPhone I wanted was not in stock at this store.
I have to buy online.
What he said should apply to all the Docomo phones online, right?
Pop my old Docomo iPhone SIM in and I'm ok??

What if it's a SIM free from Sofmap - so NOT Docomo?
Can I pop in my old Docomo iPhone SIM and it will work???

I honestly do not care about it being unlocked for using out of the country.
I live in Japan and have been here many years now.
I just care about buying a phone ASAP.

Also when ordering "お支払い回数"
number of payments?
Does that mean I can pay in one lump sum?
Or make 2-5 payments?
If I choose 2-5 payments, do they automatically deduct same date every month?

The word unlocked does not mean it can be used in another country, it means the phones is unlocked to support any usim, to use other network, so you can also put a Softbank usim and the phone will still work.

Also when ordering "お支払い回数"
number of payments?
→ correct!!

Does that mean I can pay in one lump sum?
Or make 2-5 payments?
→ Yes, you can pay in one lump sum.

If I choose 2-5 payments, do they automatically deduct same date every month?
→ Probably you can choose 2-5 payments, and they do deduct same date every month.(from your credit cards company)

I hope u can get new one, ASAP!!