Skinny man, the mountain bike aspect very roughly breaks down in 3 areas of value and think is similar to the way road bikes break down by category.
sub £350 bikes:
These are the basic end of the market ranging from the supermarket no-name ones upto the very entry level bikes from what i would call mainstream makes such as trek, giant, specialized etc.
The more you pay in this range, the more you get, but it'll always be the basic end kit. By that I mean that the general components will work fine, but wont necessarily last as well compared to higher spec models. I have a supermarket bike in the garage, and one thing i notice is the front cranks and chain rings. Under hard pedalling, i can feel the crank the twist a little. On my more expensive bikes, they dont.
I'd personally say that if you can afford it, then the £200 to £350 range is certainly beter value than then sub £200 range. For example,
this Giant bike is £300 and includes mechanical disc brakes. I have a pal who bought one a couple of years ago, and loves it. The frame is decent quality and components have lasted well for him. Thats just an example, but to me .... saving that extra hundred pounds would be worth it.
Any suspension in this area will work as in it'll go up and down a bit, but it wont necessarily last all that long. It'll also be heavy, especially compared to a non-front suspension bike.
£350 to £1000 bikes
This I would say is the sweet spot of mountain bikes and value. Its definitiely the case that the more you spend the more you get.
That extra that you get comes in a variety of aspects. The frames get better, lighter, more advanced. The components get better, lighter and longer lasting. the wheels and hubs are lighter ( makes a noticable difference ) and the bearing qualities much improved.
Also, you'll start to get into very good quality front suspension and brakes.
£1000+ bikes
I consider this where you start to see diminishing returns. The more you spend here, the less you get. Lighter components, but much more expensive cost ... perhaps just to save a few grams here and there.
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Brakes
Hugely emotive subject.
V brakes: They do a damn good job, esp in the dry. In the wet, just like a road bike, the performance suffers. In the wet and mud, then it is possible to go through pads and rims at a quicker rate, but with your uses, i dont think that would be tooo much of an issue
Mechanical disc brakes.
The first step up from V-brake ( just ). Bluntly, mech discs and good v-brakes overlap in terms of their performance. A very good v-brake will outperform a very basic mech disc brake. the benefit of disc brakes is though that they are less affected by the wet and mud from offroading bikes, and in general do offer better stopping power and to me a more confident feel.
Hydraulic disc brakes
The business. They are a considerable step from V-brakes and mechanical discs. If you go for agood brand name like shimano, then even the basic Deore hydraulic discs are outstanding in terms of braking power.
The down side to discs is that if you dont already have the hubs, then the hubs need to be disc-hubs in that they have mounting holes to attach a disc onto them. Some basic bikes may come with non-disc hubs. So perhaps an aspect to consider.
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So what do i think you should do ?
Well, you've mentioned your budget. As I already said, if you could save just a little bit more, then I think you may get more.
If its something that you'd like to keep for a while, then I would place most priority on the frame. Try and get the bike with the best frame that you can.
Mountain bikes are very modular, and its easy to add better components to them as and when your older components wear out. through all this, the frame largely remains unchanged, so out of all the bits you want to last, the frame is the most important.
So whats a good frame ? all the same things that make a good frame on a bike. Good quality tubing and welding, internal butting etc. Features to allow extras added. I'd be looking to get a frame which will allow you to add disc brakes at a later date ( if not fitted already ), that way it gives you the option at a later date.
Do I think you need front suspension ? No if its bridleways and towpaths. If you fit a large tyre on the mountain bike ( 2" to 2.3" ) , then that tyre offers a good amount of suspension in its own right. Actually, probably better than a very basic front fork ( which will weigh a lot ).
I really suggest you talk to the bike shop a little more, perhaps with these thoughts in mind: