If I'm inclined to bid NT with 15-17 points, but am weak in one suit, how weak can I be, and still bid 1NT, as opposed to bidding 1 club, to find out if my partner can show strength e.g. a stopper in my suit, by bidding it?
Your partner's response(s) ought to give you a good idea of whether a no trumps contract is the correct one to pursue, or if you might be better off looking elsewhere.
Perhaps your partner will bid Stayman over your NT bid. If you find a satisfactory fit in hearts or spades, then the question of whether you have a shortage somewhere else becomes irrelevant (in fact, it turns into a bonus).
Otherwise, your partner may well - indeed, is statistically quite likely to - try to bid the suit that you are weak in. If he does, then this reinforces your initial assumption that a no trumps contract may be a good place to be.
If your partner avoids the suit you are weak in - especially if he avoids it twice, then perhaps NT is not such a good idea. However, if this is the case then he has almost certainly bid in a suit that you hold 4 cards in, by now. If so, you have a pretty good option to shift to, outside of no trumps!
If you have a balanced hand, 15-17 HCP, but you don't open 1NT, you may encounter difficulties rebidding.
For example, you open 1m and your partner responses 1NT, and then you are in a dilemma. Your 2NT rebid promises 18-19 HCP (between 1NT opening and 2NT opening), which is unsuitable now. Sometimes you reverse with a 4-card major and your parter goes 4M with only 8 points. Other times you pass or bid a 3-card suit, but your partner has 9-10 HCP.
For 15-17 HCP, I open 1NT for all 4-3-3-3, 4-4-3-2, 5m-3-3-2, and 5m-4-2-2. Whether to open 1NT for 5M-3-3-2 or 6m-3-2-2 is another question to discuss. Don't forget 1NT opening preempts opponents' majors!
I frequently open a 15-17 1NT with a small doubleton minor, but guarantee at least Qx or xxx in each major to stand partner's possible weak transfer.
A hand that fails this standard, but does not have a 5-card major, has at least 7 minor suit cards and thus is opening a 4-card or longer minor suit.