Permanently connect the monitor to the laptop?

Is there any downside if I use a monitor as a screen for my laptop? Unfortunately, I have no PC and work only on the laptop and that also for a relatively long time. That's why I bought a powerful laptop. Only in the long run will I get a crooked back, because I have to constantly look down. I thought that can be solved by buying a monitor for around 100 euro and then set it to the correct height. Now I have a few questions about this:
1. Would that change the performance? The laptop has a Full HD screen and the monitor would be Full HD.
2. Does it make a performance difference if it gets a 120 Hz screen instead of a 60 Hz (like the one from my laptop)?
3. What about the power consumption?

In my eyes pointless and too much effort.

Of course, the graphics slow down. Must supply 2 monitors.

60Hz to 144 is absolutely overrated. You can tell when working no difference.

For 50 euro you get something good used on Ebay. Pay attention to an access time under 2ms. New ones are at 1ms.

I used to do this like this:

Notebook as desktop replacement, with external keyboard and mouse, plus an external monitor. The notebook closed while folded aside.

The only annoying thing was that I always had to open the notebook display when switching on.

As mentioned here, I think a 120Hz monitor for not really useful if it is only for the office area. A performance difference you will not notice when using only one monitor.

And how did you set that the laptop does not go out, if you closed the screen?

Should have been clearer. The monitor will serve as a replacement, so the laptop screen will not be used. And I'm not sure about used electronics. Do not know if you can trust the devices then, because of wear and so. Be sure to do a lot of research.

Can you justify that? Why pointless and I just have to buy it and then join, is not really expensive in my opinion. And since I do not want to buy a +900 Euro PC (and I would then because I then would like to gamble on it), I pay just a 1/10 of it, so my back and neck does not suffer from it. What do you think would be an alternative to my idea?

In the energy saving setting. Faith there was where to adjust what he should do when it is closed.

So the attitude is certainly, but now can't say where that was.