On the right hand console, there is the amber trouble light. Make sure it is not flashing any codes. Otherwise usual stuff. Look for leaks. Make sure the 4x4 actually engages and works. the loader should lift the front end off the ground. Make sure the PTO engages. Check coolant for oil and oil for coolant. Check the front axle oil.
How many hours? Ask when the hydraulic oil was last changed and did they do filters and clean the screen in the sump.
The 300X loader is the smaller one and the 420 replacement. the 300CX is the heavier 430 replacement. As long as you are ok with the 300X capabilities then it is a good loader. I think it is rated for about 900 Lbs.
I'm not really sure on prices these days, but seems like they are creeping up on these machines.