Id recommend renting a mini-ex or TLB or trencher
Second option would be to buy a used older construction TLB
Third option is buy a Kubota TLB vs adding a BH to an AG tractor
Fourth option would be a frame mounted BH for your AG tractor
If you really need to do a lot of serious BH work the AG tractor will get stale fast when you have to climb on and off the machine every 3-4 feet