You're asking questions for a £300 bike you should be asking when buying a £1500+ bike really. The fact the forks move up and down is a bonus on a £300 bike.
At 3-400 quid you'll get 8 speed, be it shimano or SRam - both work well, one with with finger and thumb(shimano) the other uses thumb to shift up and down. DO NOT get a bike with gripshift at this price. Google pictures of the shifters and you'll see how that works. Shimano altus/acera and sram x4 are common at this price.
Wheels....much of a muchness, will be some random alexrim on a formula/shimano/quando hub. Wouldn't look into it too much.
Forks wise it'll be some sort of suntour xcm/xcr you'll get with preload and lockout, does what it says on the tin. Heavy but it works and will lockout for when you need it to.
Brakes will be tektro most likely...auriga/draco and are up to the task of stopping well enough. Hydraulics require less adjustment and are more powerful but do need bled every year or so which you might need a shop to do.
Frame will be aluminium...pick one you like the look of as they are, again, much of a muchness. 6061/7005 series...makes no difference(to do with how it's welded and the heat treating of it).
Tyres...it's good to get a branded tyre at this price as it's something I'd want to upgrade if it was a tioga...continental/schwalbe/maxxis/kenda are brands to look for.
Cranks will mostly all be a triple set up,so three rings, but the bearings they run on can be of different standards. At this price it's likely to be a normal bottom bracket with a square taper, possible a spline(better interface) but you won't find external cupped ones at this price as they are a couple of tiers up spec wise. Brands like suntour/truvativ/fsa/shimano are common at this price. Suntour are pretty cheap £25 for a basic chainset but you can't change chainrings individually whereas the other brands you would be able to swap them out as they use a standard shared between each other.
