It sounds like you definitely need to brush hog the 3-4 acres that were cleared, or they will revert back to forest.

The trade-off is always that a job with smaller equipment takes more time.

If the previous owner took care of the tractor, then you have a good start!

P.S.
Some invasive plants/trees are not harmed by mowing and you need to use herbicides. I don't have blackberry, but a farmer near me does. I have heard him complain about fighting it.

My worst problem is honey locust, which re-sprouts and spreads after mowing. I visit the Noxious Weed Dept. in our county 1-2 times/year. They can advise which herbicide is best suited for which problem, and they sell it at cost if you own land in the county. Your county probably has a similar department.

[A few of the herbicides will kill fish in very tiny quantities, so also check with the experts before applying those in your pond watershed, or your neighbors.]

Good luck on all of your projects!