Orange make superb mountain bikes which are popular with the more "enthusiastic" rider, but if it's second hand and so cheap then I'd be dubious - there's a good chance that it's been thrashed.
Boardman make very good quality bikes with top notch components. I've got a Boardman Hybrid and it's an ideal road and light trail bike. Halfords do Boardmans and their own Carrerra range are well specc'd and excellent value for money - I've got a Carrera Vulcan and it's plenty good enough for trail work.
Hydraulic brakes are a good plus but not essential as there are plenty of good quality mechanical brakes. As an ex downhill rider I've known hydraulic brakes to fade but for normal trail riding you'll never have to worry and they require far less maintenance than mechanical ones.
I would look for 24 or 27 gear bikes, you may find 21 gears are too widely spaced in certain ratios...especially if you're just starting out. Don't forget there is a certain overlap between front sprocket ratios, so a 21 gear bike with a 3 ring front sprocket and 7 sprockets on the rear cassette in reality is more likely to be the equivalent to a 15 to 18 gear one.
A hard-tail is all you need, full suspension is overkill for trail riding and not much fun on the road. Look for 120mm travel in the forks and if they have rebound/preload adjusters and a lockout option then even better.
As Stevebrook said, avoid twist grip geared bikes like the plague - you will always end up with false gear changes. Thumb/trigger shifters are far easier to use. There are two main types - Shimano ones are two levers with thumb push and trigger pull type for up/down changes, whereas SRAM favour two thumb push levers. I personally prefer Shimano's thumb and trigger type - SRAM's can be hard on the thumbs.
Most mountain bikes come with 26" wheels, but 29" and 700c (roughly about the same size) are becoming popular. Bigger wheels = faster speeds, generally speaking. And if you're 5' 10" + then you might want to consider them.
Go for tyre sizes (width) between 1.90 to about 2.4 - the narrow ones run faster but you lose out on some side grip. Wide tyres give better off road grip but require more effort in a like for like situation against the equivalent bike on narrower tyres (especially on the road) and are best for loose/muddy surfaces.
If you're doing a mix of trail and road then I'd go for trail/road hybrid tyres - 1.95 to 2.10 wide on 29" rims.
Most new moutainbikes come with saddles that the Spanish Inquisition would be proud of. The thinking is that you'll be out the saddle most of the time anyway. I always bin the supplied saddle for something a bit more padded, but as Stevebrooke said - you've got to break your arse into riding regularly, regardless of saddle. There's no way round this I'm afraid - it'll take a few days until you get used to it. Start on short rides and build up - but the most important thing is to do it regularly.
Whatever you get don't buy cheap e.g Raleigh or those shit Muddy Fox bikes that Sportdirect sell.
Something like this is great value for money:
https://www.halfords.com/cycling/bi...era-vulcan-mens-mountain-bike-16-18-20-frames
