Russell i will vouch for TR and use 16x4 with no problems at all. I cut the thread on Acetal with a home made tap and when oiled works really well. The truth is i can replace these nuts 60 times over before catching up with the cost of a ballscrew. Would suggest you buy stainless if you go that route ;-)

Like whats been said you could alway change it later (don't forget to make allowances for it in your conversion). And yes i use ballscrews too.....