Assuming you are buying from someone that has good reviews, it should be OK.  It costs a lot of money to design and build an electronic group set and if some Chinese company has managed to build a working rim brake one, they will make more money just selling it under their own brand. 

It could be someone has just got hold of a load of old stock from somewhere (e.g. a company that went bankrupt).

You would probably be better off getting a 2nd hand 26inch, then replacing any worn parts (maybe giving a respray and getting her to choose color).  29inch is far too big and if you get a new 27.5inch, it will only have to be replaced again. 

If you want to get something she can keep for a while, you could probably get a Brompton (or Chinese clone).  Near me I have seen fathers cycling with their kids (who around 11) and they all had Bromptons.   A Brompton isn’t a mountain bike though.

On my road bike, I used to run SKS race blades (sounds like it might be the kind of thing you are after), but they limited my tyre size too much (might not be a problem for you if you have lots of clearance).  So now just use a generic straight rear mud guard which attaches to the seat post.  This is enough to keep water from splashing my behind with cycling in rain.

On my trifox gravel bike which I got recently, I am not using any mud guards though as I prefer the look without them. 😀  Plus if you are going somewhere muddy, you are going to get mud on you whether you have them or not.

That hydro adapter is a lot cheaper than I thought it would be.  I was looking to upgrade a family members cable brakes recently with some cable/hydro brakes, but it was actually cheaper just to buy the hydro version of the group set off eBay (brakes and shifters). But looking at that price, I might just go with that.  It is a gravel bike, so they already have a lot of things attached to it, so shouldn’t mind just sticking those things under the handle bars.

Might consider buying since locally SRAM is more expensive, just wonder if it would work with Sram K7/Cranks/Chain because I wanted to go 10T

How is the crosscompatibility between Shimano/Sram, can this groupset work with any 12v K7 from them? Also its interesting that I could use the Shimano caliper instead of Ltwoo

Shimano's upcoming gravel Di2 is meant to be using micro spline hubs (to get 10T), so that might be a better option for those wanting 10T with ERX (use micro spline wheels and cassettes), as the chain spacing should be the same as standard Shimano HG (I believe XDR chains have slightly bigger gaps)

There are people on youtube that are using Shimano cassettes with AXS and the other way round.  So I would have thought ERX would technically work with XDR cassettes, but there have been reports that the shifting isn't quite as good when mixing AXS with Di2 (12 speed). 

IMO the best 2 good cameras for recording bike rides are:
-GoPRO 11
Best stabilisation, I used it on my MTB handlebars, cycling through the local forest and the video is so smooth it looks like I am floating. 
-Insta360 X3
Takes 360 degree video, so you can focus in on different parts of the video when editing.  A bit like switching between different camera angles.  I see a number of commuters using cameras like this as it is great for capturing footage of dodgy drivers. 

However if you get a camera that is supported by Gyroflow (free software), you can get similar stabilisation to the GoPros with a bit of extra work. 
https://docs.gyroflow.xyz/app/getting-started/supported-cameras/runcam

For example the Runcam Thumb Pro-W can generally be had for less than £100.