PT-AX200U...a oldy but a goodie.
Many are the instances where a really good, high performance ambient light application has centered around that unit.
Straight away, I suggest you consider Silver Fire v2.5 4.0
applied directly onto the Sintra using this inexpensive HVLP sprayer. http://www.gleempaint.com/noname.html
Here are shots of that PJ and a similarly hued Silver Fire Light Fusion at 120" diagonal (w/new Bulb) at a 13' throw distance.
High ambient light is not as big an issue as is light directly washing against the screen from a source, be it Sunlight, Incandescent, or either as is reflected off a nearby surface. With a Screen on 48" high, and with the Panny's Lens Shift, you should be able to place the Screen as low on the wall as possible, therein avoiding reflective light off the Ceiling....your biggest troublemaker. (...especially during dark room viewing...)
You don't mention as to how you want to mount this Fixed Screen, but the price you stated indicates to me your getting the 6 mm thickness. Lots of options on here to choose from. Your going down a well traveled road with a great many experiences having gone before you so no worries as far as the unknown.
I would add that since your starting with a screen made from 48" x 96" Sintra (ie: White Fusion ) , that you might consider the epitome of Ambient Light Screens, Light Fusion. Purchasing a 4' x 8' x 1/8" (3 mm thick...) Acrylic Mirror would allow you to go further out to a darker SF 5.0, and give you the best performance possible due to the higher reflective index the Mirrored backing will provide. Painting a Light Fusion has only become all the more easier than ever using the darker SF versions and the new Electric HVLP Guns.
I'll tell you this....if the Mirrors were still available in the 5' x 8' and 5' x 10' x 3mm size, Sintra would still be waaaaaay down on my priority list. There is that much additional potential. You might want to check that Plastics Dealer out as the West Coast is one of the few places that Display Makers seem to consistently have need of the larger Mirror sizes. With a 5' x 10'er cut to 54" x 96" you could easily go out to 110" diagonal within your Throw and seating distance parameters. Certainly you will be able to easily get a 4' x 8'er and have them cut off one end to get the 85" length. Let me know if you need any assistance as to getting the best "mirror" price.....but as far as the best price for the Sintra, your already there.
One bit of important advice. Due to the design of the Pannys "Jot Stick" type Lens Shift control, try your very best to mount the units with the Lens as centered on the Screen as possible. Also, give yourself another 3" - 4" throw distance if possible if you do decide to go with a slightly larger Screen size. (...at 98" diagonal 11' Throw is just fine....)