How much should I spend...?

[Tamiya Cowboy]

You can do the Bruder dozer for 300.00 I used the tamiya gear box that small haul used it works great and pushes very well for a grading dozer all you need extra is the right ESC and 2 good micro servos with lots of tork ..

As for the Bruder skid steers I built the tracked cat and this will cost you over 400 there are a number of spendy servos in that one if you do it right it is adds up fast..

Running the RDS8000 in both of these units I am able to run both with out turning them off just but switching back and forth..

You can do it I would save up to 400 to get started and you can get a great remote and have a well built unit right off the bat that you wont have to rebuild down the road do to cutting corners just to get it running..