At 18 PTO HP, you are on the low end of using that 5' cutter, but it can be done (I did it with 16.5 PTO HP).
It really depends on what you're cutting. You'd have to make partial passes on the thicker stuff and probably go pretty slow.
Transporting the cutter raised would be tricky (cutter weight would be close to 50% of your tractor's weight), but you can always tow, rather then transport.