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Well the first thing is to identify the shortest and longest useful focal lengths and work out a selection of say 3 or 4 focal lengths spanning that range, then look at what's available.
An f/5 that suggests you might have the occasion to use 4-5 mm, while at the long end around 25mm is likely to be the practical limit. If you're opting for say 3 eyepieces, this suggests 5, one around 10-12 and 25mm. For a set of 4 eyepieces: 5, 8, 13-15, 25.
Note you could also do something similar with 1 eyepiece and 2 Barlows, or 2 eyepieces and 1 barlow.
There is also the question of what you are using it for. For high power I would suggest image quality (sharpness edge to edge, colour correction especially near the edge, transparency, distortion) are higher priorities than extreme field of view, and in this respect you could consider thiongs other than TV Naglers/Ethos, such as the Zeiss Abbe Orthoscopics.
If you want the ultimate widefield experience above all else, then yes the Ethos/Naglers are for you, and you'll only need 2, possibly 3 to cover the entire range and FoV of your scope. The Explore Scientific 14mm widefield is an Ethos clone, and should cost a lot less than the Ethos.
Lastly you didn't say anything about your vision - I wear glasses - and I find it a lot more comfortable to use eyepieces withn a lot of eye relief. For this reason I have a set of Vixen LVW's and a set of Vixen LV's - the LVW's are "the good ones" and the LV's are for when I have other people around or star parties etc.
My scopes are f/7 and f/15 so I can go right up to 50mm (though this would be useless in yours):
LVW 5, 8, 13, 22, 30 and a TMB Paragon 40mm,
LV 4, 7, 10, 15, 25 and 50.
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