For cameras I have no major brand loyalty, and so many of them are clones - often with the same underlying chip.
My favourite was the Contour, which is tubular and is compact on the side of a head/helmet, much better than a GoPro box on the side. Contours have their own style of mount and mine also has a standard thread.
GoPro won the action camera war and the market is full of GoPro style nowadays and that’s where I’d say to go for a general use action cam.
SJ___ cameras tend to be good, but any generic brand measures up these days.
When someone’s got more serious and knows what suits them then they can look into what type/quality the sensor is, how good it is in particular modes etc and how well it performs in sunlight, indoors or whether it needs to be best at coping with going from indoors to out with the change in condition.
Nowadays pick a 4K capable camera that is at a reasonable price.
For accessory packs I would tend to go for these types:
www.amazon.co.uk/dp/B01CSB2M2C/ref=cm_sw_r_cp_api_i_dlC_5LNWFbG30AQ48?tag=mumsnet&ascsubtag=mnforum-21
You would be hard to find any brand difference between these accessory sets, some of my camera accessories have been CamKix branded.
To pick a selection, I would go for a range of different types of mounts and lots of the ‘arms’ that give choice. If it’s going to be put on and off many different things then opt for sets that have lots of spare 3m adhesive pads. When using the pads and particularly if being ‘active’ with a camera then always tether it as well, even if it’s just a piece of cord tied to the camera and the object, so if the mount fails the camera hangs on instead of flying off
Search you favourite online shop for ‘action cam accessories’ and you should find a few selection packs under £20. You could pay £10 to £20 for a single mount and these packs aren’t the finest quality but they give a good start, plus sometimes quantity helps a lot to solve the mounting problem of the day. I’ve gathered a number of packs which means I have a bag full and can put together some form of mount such as attaching a camera to me, a bike, skateboard, car windscreen, a helmet, a wall, a tree etc or even just putting an action camera on top of a stills DSLR*