You really have to say how much you want to spend.
It's a 10 year old machine and you said 'to replace your worn MBP' so why do you think it'll be much less worn out than yours is?
It depends how many hours it's been used of course and what temperature the inside of the machine has been running at- the cooler the better and that does make an enormous difference- but realistically:
It will need a new battery if it hasn't had one. Even if it has had one it might still need a new one
The screen? Light use clean environment- 15 years.
Moderate use maybe 12 years?
Heavy use, lots of opening and closing - 10 years
If it's dimming, flickering, odd colours round edges, vertical lines, artefacts, it's at the end of its life
Fans will be beyond their expected life which is about 50,000 hours. They are expected to last only about 6 years of moderate use but they may last a couple more years if it's been lightly used.
The solid state capacitors, will likely have 2-5 years life left in them if you are lucky. You'll probably get 2 more years out of them on average.
SSD will be slower than it was but should last a few more years. HDD is at the end of its life. Failure rate now will be high.
CPU should be fine for a few more years.
Logic board should be fine for a few more years.
USB, and other ports- likely some physical damage after 10 years
Keyboard 2015 MBP rated at 5-10 million keystrokes per key- which is a durable keyboard. It's probably at the end of its life but if it's been lightly used you might get 5 more years.
You are now in, 'anything can fail at any time' territory.
Realistically, if you are lucky, you'll get 2 years out of it unless you know for certain it's been well looked after and lightly used.