TV mounts are specced the template shape on the back of the TV (called VESA mounts) that the screws fit in which is normally defined as a square or rectangle, so say 200 X 200 or 120 X 80, and a maximum weight. Most mounts will fit any TV that meets with the mount's minimum and maximum sizes. So a TV mount will say 'Anything between 60 by 60 and 200 by 200'. if you can find a mount that fits both what you have now and what you plan to buy, you're OK.
I'd recommend finding the screw holes on the back of the TV you have now (assuming it has some) so you know what you need now and then, considering at the size of TV you plan to get, check the specs of a few models which will tell you what size mountings they need. If you can find a wall mounting that will work with both you're good to go.
I have mine mounted on the front of the chimney breast. To be honest, if it's a reasonably old house, unless you're using absolutely massive screws to put it on you're not going to be penetrating through the brick. And even if you do, the chimney should be lined anyway. I wouldn't worry about it.
Mine's quite high and it's not been a problem. Somewhat above eye line is probably best as most mounts tend to tilt the TV downwards a bit.
Not really. A the end of the day - unless you're going to go for sophisticated stuff like something with an articulated arm that allows you to wave the TV around - they're just bits of metal you screw to a wall then screw the TV on to. Mine's fine and cost me like 25 quid.
One tip - before you spend the money, check you can take the stand off the TV you've got. if you can't, it's a no-go probably.
Jon