Just curious. What would I find different in using a $500 to $1000 unit, as opposed to the inexpensive Wagner?
Looking at a correctly finished canvas made either way, probably nothing!
But for volume work, there are big advantages to a professional unit with a separate turbine...
The Wagner's tiny filters are located too close to the nozzle and clog quickly. You have to clean them pretty often to keep a reasonable air flow available. (One work around is to cover the inlet ports with a layer of the air filter stuff made for home heating systems, rubber band it on.) But with a separate turbine you can locate it upwind or in another room and the filters stay clean for a long time...which means a consistent, predictable airflow, always.
With my trusty Fuji, I can spray a one foot or 300mm wide, heavy swath of paint at almost one square foot per second (takes practice!). It makes a difference when I've got 3, 4x8 foot panels of prints to paint and am also hammered by other chores. Takes a gun with precise nozzle and pattern adjustments, which the Fuji has. The Wagner has only a one-size-fits-all adjustment and can't provide that much volume or width. And the Wagner would be significantly filter-clogged after the first panel, that's my main gripe.
But honestly, if I only had a Wagner, I could probably stagger through heavy production somehow, although I would be quite a bit more grumpy at the end of the day than if I were using the Fuji.
As for having trouble rolling largish prints, amen to that! But usually the problem is that you're not getting paint onto the canvas fast enough. You have to pour the paint right on to the canvas, then roll it out. You can't transfer paint onto the canvas fast enough with the pan-to-roller-to-print technique. Also if you put down really thick wet coats that buys you the time you need to get a large surface coated before the paint starts setting up. Most rolling problems come from the paint going tacky while you're still rolling, so wetness is you friend.
As a ballpark figure, calculate the entire area you need to roll, which includes a few inches on either side of the prints. Measure out about 0.004 ounces per square inch of coating into a cup. That's about what it takes to get a nice, wet surface. Oh, and if the roller is dry, you will need another 2.5 ounces (75ml) of paint which is what my 6 inch soaks up, your roller may vary.
Part of the art of rolling is to figure you how long you can roll without leaving marks that won't level out. You can go just a little bit past the point where the paint starts getting tacky. You might want to do this kind of extended rolling if you get pinholes, you need slightly tacky paint to fill in persistent pinholes on a single coat. You need to use very little pressure towards the end.
I can think of no human endeavor which benefits more from a little in-person instruction than canvas rolling. As Dan mentioned, he could probably get just about anybody coating canvases correctly with nothing more than a few pointers and couple sharp slaps across the face!