I want one.

[chas]

I've always fancied building something along those lines Rod, If I ever win the lottery I might just get rounds to it, don't hold your breath.
A nice quiet engine, well sound proofed, not bothered about high speed but must have rotating drop axles to alter the ground clearance, hydrostatic drive, tyres not as big as those on the Russian job but must be low pressure with soft sidewalls (after all that's the only suspension you'll have).
Adjustable track width would be nice to fit in tram lines.
Front opening window to poke a rifle out of, now to get really silly, a rotating glass dome on the roof with rifle slot.
Good heating system and comfy seating would be a must, a bed for all nighters would be good
Oh look! I've woken up now and the dream has faded.

[rodp]

Hydrostatic not a good choice. Trouble with heat dissipation, very poor efficiency, expensive to build and maintain. Chain and disc drive is far better. Duplex 60 chain with hydraulic disc braking is simple and cheap (Relatively, Argo chains app £400 / £500 set).

You could build one easily and cheaply over time, give yourself say 2 or 3 years to acquire parts and build. Tyres would have to be plant tyres with the rubber between tread cut away with a tyre cutter to allow them to flex. Quite a common thing on some off roaders. Any front wheel drive engine could power it, just weld sprockets to drive shafts and run it through a jack shaft to lower gearing.

That's your plans sorted, all you got to do now is build it

[chas]

Going to have to agree to disagree on the hydro bit Rod, agreed not as efficient as direct drive but modern systems more than capable of handling this sort of thing.
Some combines are hydro, as are telehandlers, crop sprayers etc.
The big advantage of hydro is that you can move things about without any hassle except for hydraulic hoses which can bend as required, so rotating drop axles and variable track width are not a problem.
Would love to be able to have a go but not expecting it to happen any minute now.

[rodp]

Big problem with hydro is heat, to do any sort of speed you need a cooler and to do anything approaching 20 / 30 mph it will need to be a relatively large cooler. That's expensive, fragile (relatively on a vehicle with no suspension) and a pig to site in an airflow. The other problem with heat is it's just wasted energy, every degree of heat is power lost.
Take a look at Mudd - ox atv's, they're hydro, they've taken a long time to get right but they've done it. Very expensive though compared to Argo and not something you can repair in the field with a hammer and some bolts.
Hydro isn't the way I would go unless it was purely an engineering exercise, keep it simple, strong and economical