If you aren’t looking to build your own models or just looking to easily expand your collection, there are quite a few radio controlled vehicles available off the shelf. The Siku Control 32 series is very good and I highly recommend the Orlandoo kits which allow you to assemble your own top quality 4WD RC jeeps.
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The Siku Control 32 range are the best off the shelf RC tractors you can get.
Siku make a good range of static diecast models which are quite affordable but still have good quality.