If you don't already have the PTO installed, you might want to check out the price. Also check out how easy/hard it is to install. There are some threads on it here. Some people have had some difficulty installing the PTO kit on an older tractor. The 3PH is also a longish install.
My neighbor has a 42" rotary cutter. He has an older L size Kubota. I tried it on my B2620. It handled it okay, but it felt really big and heavy back there. You will definitely need serious front weights and/or a loader to even it out.
How big an area are you planning on mowing and how often?
What is in the area, is it pasture with just grass or do you have lots of brush or trees you are trying to remove.
People do use the X700 series tractors with a 4' rotary cutter, but I would not want to do that for long.
The PTO and 3PH cost about $1,700 + shipping + install + tax. The rotary cutter costs at least $1K unless you can find a deal on a used one. This:
Swisher 44'' Rough Cut Trailcutter Mower | Cabela's
is $2500 + shipping + tax
The PTO/3PH rotary cutter will tend to hold its value well, especially it you take care of it. The PTO and 3PH will make your X738 more attractive to potential users, but I would say that would be in the $500-$1000 range depending on the condition of the tractor. If you plan on keeping the X738 for many more years and you expect to get a lot of use out of the 3PH and PTO, you should get them. If it is just for the rotary cutter, I would think at least three times about it.