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I wonder how a plastic tank model can be such pricy!
Is there any cheap alternatives in same size/style?

Something like a 1/35 Challenger would be your best bet.
The decent kits from Dragon/Tamiya are still fairly expensive though and your extra hours and cost of bits spent converting to be fit for purpose will soon eat into the difference in price, if that's a consideration.

If I remember that right, you may use a scratchbuilt vehicle, if it is of adequate proportions…

If so, you may scratchbuild a superheavy vehicle by using the appropriate templates (there are some made by this Eli Patoroch guy, which are wonderful). Just download, print and use to build on cardboard/foamboard/plasticard.
If you take enough time for detailing (rivets and so on) the result may be better than the original... just google for them and you find anything.

And how I will do tracks? That's the real problem and I'm asking for something similar. Maybe toy tanks

The main problem will be the size of the guns (as GW ones are well over-sized) followed closely with the much finer panel lines & rivets you get on 'proper' scale models.
Toy tanks may be the way to go, as you stated.

You can buy tracks seperately. Wazzoo2000 wrote about them and posted some links in his blog here. But ordering something from some obscure GB hobby shop to be delivered to RF? That may take more time than learning how to carve tracks yourself. Also, wazzoo made a tutorial on grot tanks, treads and all.

Tracks ahoy