I don't think you should listen to the art dealer. Does he print pt/pd? There are many brushes out there, and many that may be suitable for pt/pd. However, one that we KNOW of that works REALLY WELL is the magic brush.
I have heard comments of a Simmons brush that some people like better, but I don't have personal experience with it to recommend it.
Dixie is the best place to get the brush in the US. I don't know how that compares to the prices you found in Canada.
I use a 2" brush for 8x10 and 7x17, and also for 11x14. Bigger than that, I use a 4" brush. I would be concerned that the 2" might be too big for a 4x5 print. Luckily, the smaller brushes are relatively cheap. Remember, as long as you clean it out properly after each coating session you only need on brush in any size, since they are worked totally wet.
One recommendation, coat up multiple sheets at a time. This will be more time efficient, and also will eliminate the problem of dilution after the first sheet of coated paper. But, you may notice a slightly different quality of print from the first sheet to subsequent prints.
Then, after you have coated four or more sheets, you can wash the brush and the other items all at once. When I'm on a roll, I never coat one at a time. I'll often coat 4-6 sheets and then clean, then recoat the sheets, blow dry, and then print. I think it provides more consistency and is more efficient to work this way.