No, it's not too heavy, and it's about a perfect match for your tractor if your doing heavy cutting, or want a cutter that will last your lifetime. I have a very comparable brush cutter (Brown 472) and the same frame size tractor as you have, with less PTO hp. It's an excellent match.
As mentioned, you need to either keep your FEL on or use quite a bit of front end weight. When I'm mowing in the woods (about the only thing I don't mow is nice clean fields) and I want to take the FEL off for maneuverability, I leave top link in "float" so the rear wheel of the cutter is always supporting weight, this way I don't need any front end weights.
When I lift it clear off the ground, I can get the front tires of the tractor in the air if I stomp down on the HST pedal, so yes it's a lot for the tractor, but not too much.
I imagine if your looking at a BH 326 mower, your very much aware of the importance of quality. :thumbsup: This mower would last a lifetime.
However, if your mowing field grass, the BH26 would do everything you need as well. I tend to recommend "overkill" so keep that in mind.