I've been sandblasting antique & classic cars and parts professionally for the better part of thirty-four years and I believe I can properly advise you..
Question #1 a 10hp, 120gal set-up.. Are you going to be sandblasting Aircraft carriers? I have, and effectively use a 5hp, 60gal (twin cyl, two stage) set-up and my pressure blaster has never "out run" the compressor. In other words, my compressor will "cycle" off and on during constant blasting.
Siphon Feed blasters are NOT worth the powder to blow them up! Most require 90psi just to work, and the "quality" of their "work" is laughable.. A Pressure Pot blaster, on the other hand, can work very effectively at 50psi (I also dial it down to 40psi for the more delicate items!), remove the toughest baked-on enamel & rust and NOT "heat-warp*" the sheet-metal (*If you "bore-in" on one spot, you WILL warp the metal and probably cut a hole in it!)
Question #2. I have a cheepie H-F Blast Cabinet, I use my Pressure Blaster in it, as well. The small size of the cabinet is its only limitation, but I bought it for small parts and it works great.. After I run about 50lbs of sand into the cabinet, I "pull the plug" at the bottom of the unit, drain the sand and reuse it. (in reusing the sand, it gets beat finer and finer, great for putting a "satin finish" on interior garnish moldings on '50's era cars!) I paid about fifty bucks for the cabinet nearly ten years ago. I've replaced the viewing window once (I tape-on clear 0.010" Lexan sheets to protect the inside of the viewing window.. Each Lexan sheet lasts about an hour.. and so does fifty pounds of sand) and I've replaced the "floodlight" bulb once.
I've never used anything but "#4 Blasting Sand" for both "open" and cabinet blasting.. Where is it written that you "have" to change media when blasting in a cabinet?
I have the plans for a PA Blaster.. If you can't afford a decent quality 100lb Pressure blaster, then maybe using the PA design on your siphon-feed blaster would be an improvement.. Given that I use "Low Pressure, Low Angle" blasting techniques effectively, the PA is not for me..
Question #3. The Advantage to a "Blasting Room" is a simple one.. it allows one to "recycle" the blasting sand.. There is method to this madness.. As noted above, I use #4 Blasting Sand.. it is now the "finest" grit available in north-central Texas.. thirty years ago I used #5, it was discontinued. I buy 20 100lb bags when I ready to blast a car and all it's bits and pieces.. With the "new" sand, I blast the Chassis (frame) and all "hard parts" like suspension, steering rods, etc. I refill my pressure blaster with a new bag of sand until all these "hard parts" are done.. Then I sweep up the sand on the floor, strain it (thru a mesh screen) and put it in a clean barrel (used only for sand). I then take the "next batch" of parts to be blasted.. not quite as "dense" as the frame, etc. but still "heavy", and I use the "once used" sand. I continue reusing a previously used batch of sand on each "lighter gauge" batch of parts. By the time I blasting with "powder", I'm blasting the body, doors, hood (all the sheet-metal) of the car. I should also note that immediately after each "batch" of car parts is done, it is blown off, wiped down with a Degreaser, tacked-off then shot with an epoxy primer. No rust is allowed to form.
My "blast room" is a corner of my shop that is "walled in" with heavy mil clear plastic sheeting (I buy it in 100' rolls). The "walls" are all clear plastic and one continuous piece, the "ceiling" laps over the walls. The whole mess it attached to the ceiling rafters with 1x2's and wood screws. Allows for easy install and take-down. I use 2x4's to hold down the bottom of the walls. It goes without saying, but a respirator (not a dust mask) is a MUST when doing this.. also guard against dehydration, as well (the temp can go over 135 in this plastic enclosure during a "normal" Texas summer!).
Lastly, Question #4. I bought my Pressure Pot Sand Blaster at an automotive swap-meet from an Oklahoma fellow who, I believe, built these units in his garage. Seems I gave a couple hundred dollars for it, but I got a longer than "standard" blast hose (helps in moving around a car body on a rotisserie). It has a 100lb capacity-- 110 if you "stuff it" full-- (that's about 60-90 minutes of blast time), replaceable ceramic nozzles (about $5 last time I bought them). I bought this unit just about twenty years ago. If you are familiar with "TIP" blasters, it's made pretty much the same way.
Bottom line is, you don't need "bigger and badder".. Sandblasting is an Art, not a Demolition Derby.. a light touch, finesse and "quality time" will result in a high quality job!
I hope this answered your questions as well as clearing-up your misconceptions about sandblasting..