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Marcel,
No don't fill the pipe with concrete, for two reasons: the point of using coarse river sand is that it damps rapid vibrations very well, whereas solid concrete will resonate ("twang") like a tuning fork. Secondly when the time comes to dispose of this (possibly before you sell the house) its easy to empty the sand out and the pipe can be carried away. A 100mm x 3 metre pipe full of solid cement will be a bigger problem to dispose of.
I made mine with timber from a timber yard with little more than a circular saw and a drill. I just looked at how a typical deck is constructed and copied that, the result was quite cheap and even now would be a few hundred $, max.
Aah yes, just remembered one specific feature you will need - I had a very strong handrail on one side - made from 6" x 2" hardwood bolted with 2 12mm bolts to the vertical posts, with one end extended out a bit past the end of the platform. I had a block and tackle that clipped on this with snap shackles and a camlock clamp to use as a hoist for lifting heavy loads up/down like the mount, rather than attempting to drag it up the ladder. This is far safer than risking a mishap with expensive optics.
I used yachting pulleys and decent ropes discarded from my boat (I was a keen sailor), you can find these at any ship chandlers.
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