The minimum that you should get for your BMPCC is a sunhood for the display, since it is impossible to use in direct sunlight. There are a lot of people that build a DIY version using some cardboard / plastic and rubberbands. If that is not your thing, there are inexpensive sunhoods that fit the 3,5“ screen.
Display Loupes only magnify the BMPCC's built-in LCD screen. People commented that the quality of the screen itself is not great, so magnifying it does not change that fact. But it's commonly considered impossible to use the BMPCC's LCD for focusing without ANY kind of aid i.e. in sunlight, both to cover the outsides light and magnifying the small 3.5” screen.
Electronic Viewfinders usually consist of an LCD Screen with a Loupe attached to it. Almost all of the available options are quite pricy. Some of them have a much higher resolution display then the BMPCC's and almost all of them can display additional information like Audio Levels, Histogram, Focus Peaking etc.
The decision for field monitors is usually based on the budget and a general decision on what size you want to have. A 7“ field monitor can make your rig quite big, so a lot of people opt for a 5” screen.
Generally higher resolution & more brightness directly results in a higher price. As of 2016, prices started to drop for high resolution external field monitors. There are two FullHD 5“ field monitors for under 500€ available now, more probably to come soon.
The higher quality brands include:
There are numerous brands that offer the cheaper (but often lesser quality) solutions like