First question is why would you want to get rid of the holly? It takes many years for holly to be come mature enough for harvesting. I'd be asking around the local garden stores to see if they needed a supplier for holly. And when you say "volunteer brushes," I'm assuming you meant volunteer plants/seedlings. And these could be viewed as an asset depending on your local soil conditions and how easy these are to dig up and pot for sale or transplant.
As CBF stated, we need much more info, about your property and your intended use, in order to give you relevant opinions.
Also, I'll have to assume you are storing the tractor inside or under cover of some sort. A bigger tractor will of course require a bigger space.
Even without any more info, I think the Massey 1715 - a good little estate lawn mower tractor, horse stall cleaner - would not be your best choice for a 5+ acre wood lot. How mature are your trees? Do you heat with wood? Are you considering using a
chipper in the future? Do you ever have to plow snow, groom your driveway, or pull cars out of ditches? And how established are these blackberries you want to get rid of?
When i first got my property, it was over run with blackberries, 6 feet high in some places, and no way of knowing what was at ground level. For clearing those areas, i just rented a small bulldozer for three days.
Hold this, as just opinion: But there are stages to a property. Particularly if it is raw land or if it is neglected land. First stage, you are going to need some big guns to get the job done. Rent these big guns. Then, there is a middle stage that you will want to make relatively small changes to your property. A 14-25 HP tractor, with FEL, and Box Scraper works well in this stage for making patios, and paths and moving materials.
Then, there is, after everything is established, a maintenance stage. The mid-sized tractor 14-25 HP will still work, yet it will be slightly cumbersome if you need more precise landscaping work.
So later, we just bought a garden/lawn tractor to do that work, mostly mowing, while still keeping the 14 HP Compact Tractor to deal with the odd heavy lifting, such as pulling cars out of ditches, clearing snow, hauling firewood, or dragging out the occasional fallen, mature tree.
If I had any advice, I'd say get a tractor with a wide stance. Even though you say you have just a slight down slope. First few years, had mine on three wheels, more than i want to say, till I learned to respect how the machine worked on even small slopes. And your ROPS is only as good as when you are wearing your seat belt.
