Hard blade use should be done with handle bars, harness and deflector with limit mount. The harness is adjustable to user so height is not important.

This is a safety issue and how Stihl expects brush cutters with a hard blade to be operated. If your dealer suggests anything else he is violating his dealer agreement with Stihl.

You really should go with a bigger machine with that much land AND doing that much brush. An R-55 would not be up to the job IMO.

Nevada makes a good point about the harness regardless of the dealer agreements. A harness and bike handles for brush cutting is a safety and comfort issue. You'll want at least an FS90, but given how much land you have and that this will not be doing trimming, you might want to consider the 250, plus hearing, head and eye protection (forestry helmet).

Features you'll want are a solid shaft, ability to take a blade and handlebars.

Don't overlook Shindaiwa. I've been running one of their C-35 brush cutters for over 20 years.
If you are going to be using a blade I think bicycle handle bars are a must as the blade will try to kick out of the cut on occasion.

Whatever you get, bite the bullet and spend the money on a good commercial unit as they are usually built tougher and will last longer.

Also drain the fuel and run the engine at idle until all fuel is out of the carb before storing for any length of time.