About those Kubota EA300s you found for $300-$500, that sounds like an excellent price, especially if they're in good condition.
Kubota tractors usually lasts a long time and works well.
I got myself a 2007 KUBOTA L3940HSTC tractor for around $29,500, a good deal. I needed a new engine and found a Kubota V2403 engine on this website. It cost me about $2,500, which was reasonable. This site has lots of different Kubota parts and tractors. You can check it.