You only need a few 5C collets in very common sizes to hold end mills (3/16, 3/8, 1/2, 5/8, 3/4, 7/8, 1 and 1.25"). There are a few more in-between sizes that fit carbide end mills, which come in shank sizes that are not normally sold in HS. I suggest you buy some old Hardinge collets on eBay, trying to avoid visibly abused ones. Older Hardinge 5C collets will not have internal threads, except for 1" and up that are still made without internal threads as an option. Time was, Hardinge only made accurate collets. The idea of selling "accurate" collets for extra cost is a relatively recent marketing trick.
You also have to consider how accurate the Darex spindle is. Start by measuring the runout of the collets you have while each is holding an end mill in the Darex.
Darex sold special 5C end mill holders with a set screw for 1.25" shanks, needed because 5C collets with through holes stop at 1.125". I think I have a spare if you need one.
Schaublin (Switzerland) still makes 5C collets that are probably as accurate as Hardinge, but I bet they cost more. I would not expect much of any other current makes from India and China. A few decades ago, there were some good 5C collets imported from Germany with Grand and Neal brands on them (the USA importers).
Larry
