Arnuld, the Meridas all seem ok and the frames look pretty similar, I'd be looking closely at the specs on each. For this you need to know what you can easily get hold of in the way of spare parts. Things like brake blocks, brakes disks, tyres, tubes, pedals, chains, chainrings(the cogs by the pedals), cogs/cassettes(the cogs at the back wheel).
I've never used disk brakes so I'd be comfier with rim brakes and it tends to be easier to get spares as well. Disks can be expensive if there's a problem.
Unless there's a physical reason not to you should go for 26" wheels.
Tyres - big knobbly tyres are hard work on tarmac and only really needed if serious offroad. If it's just loose soil and gravel then I'd keep the amount of tread to a minimum.
Bars - add on bar ends or get a trekking bar to allow you to change hand positions.
Chainset(cranks, bottom bracket and front rings) - make sure they're all seperate, ie replaceable individually rather than as a complete unit. for example, if it has 3 rings at the front you'll find you do most of your riding in the middle or outer ring. Much cheaper to just repalce that when it wears out rather than the whole thing.
Your dealer should be able to sort out the little bits above at no extra charge. Make sure he throws in things like a pump, spare tubes, onroad toolkit/saddlepack and a rack if you need one(easiest way to carry stuff). If you don't know how to do the basics(I certainly didn't) like changing tubes, repairing punctures, adjusting brakes and gears then get the dealer, or a local shop, to show you how. Oh yeah, cable/zip ties, get some and carry them, amazing what can be fixed temporarily with these.