Welcome to IPD! I've been doing fantasy football for about 28 years now and each year has been IDP so to me there is no other way.
You are right there are a lot of variables that go into when to take IDP players like league scoring, how many players, are you in a keeper or dynasty league, etc. I prefer a tiered approach to all of my drafts as a guideline but always use commons sense to avoid being on the wrong side of a run.
You should examine the scoring by position throughout the league and see where the best of each position scores, where the average of each scores, and where the worst starter scores, then compare those values. Years ago I was in a league where people didn't take IDP's seriously and they would be drafting 3rd and 4th WR while prime LB's where still there. When people started realizing that the Hardy Nickerson's and Ray Lewis's of the world were outscoring those WR's then the league eventually corrected itself. But it was good while it lasted.
Here are some observations that I see:
Look where the top defensive scorers would be and draft accordingly. It is always great to have a stud at every position. To me, Luke Keuchley and JJ Watt are worth early picks because they are young, consistent, in systems that benefit their statistics, and not in much danger of competition from their own teammates (Jadeveon Clowney notwithstanding). So if you had a choice between let's say a Torrey Smith and Keuchley or Watt? I'm taking the defensive player every time.
Speaking of systems, here is a rule of thumb: teams with the worst offenses produce high scorers on defense. Fantasy is about the best stats not the best players. So when you have a horrorshow of an offense like the Raiders, guys like Nick Roach and Charles Woodson put up good numbers. Think Kiko Alonso (dang! I have him in three leagues and can't hold him over anymore!) These aren't the type of players that will be taken early, but if you carefully examine the teams as a whole, it isn't too difficult to stock your team with players that have great opportunity to outscore their draft positions in the back half of the draft.
Pick up defensive linemen early. They are the scarcest position in all of fantasy football. Consider that JJ Watt is a freak of a 3-4 end with only guys like Cameron Jordan being close to stats. Then consider how many teams have changed to a 3-4 and subsequently eliminated the rush end in favor of a rush LB. Guys like Clowney who are ends to Buddy Ryan are OLB to guys like the Harbaughs. I would say that in a re-draft league I would try to acquire one stud LB and one stud DL by the time my starting offense was drafted.
Rookie Linebackers. It seems that every year there is at least one stud rookie LB that emerges. In the past three seasons respectively it was Luke Keuchley, Lavonte David, and Kiko Alonso. They all finished with big years and could be the cornerstone of a defense for years providing health stays good. I drafted Alonso early in three leagues last year and I only knew 2 things about him: he was an every-down linebacker and he was a rookie starter. So you should take a shot at a rookie LB because one blows up every year. This year's crop has plenty of guys who could fit that bill.
Every Down Linebackers. There is a great list that goes on the IDP forum and it helps a lot. Seems simple when you think about it and I don't know why many people don't think about it. LB's that are on the field more get more stats! I love looking for LB's that are good in coverage as they stay on in sub-packages. Last year that is what drove me to Kiko Alonso; but Danny Trevathan, a very inexpensive guy who blew up week one, also had this characteristic and was snatched on every waiver wire that I know of by the end of the first Thursday game of the season. Guys who are good in coverage are out there more and make tackles not in the run game. If for some reason you have a rush LB that is "every down" that is a good thing, too.
Rush Linebackers. This is tricky but can be a big thing. They are typically every down LB's but don't go into coverage. So they need to bring it in a big way. I find these guys are good in the like the 3rd or 4th spot and can really help out with big weeks. Think Aldon Smith, Von Miller and Terrell Suggs. They may not score every week, but every now and again they make the old sack, forced fumble, fumble recovery, and (sometimes) touchdown. Big plays like this from the back half of your starting lineup can win games and win championships. But I would caution you not to load up on these guys too early. Every name I just mentioned has had injury or off-the-field concerns. It seems like the rock start linebackers sometimes live too much like a rock start and that catches up with them. But if you can snake out one of these guys later they can blow up. I remember years ago I had James Harrison pegged in every league as a guy that "if he produces in week 1 then I was going to jump on him everywhere." He had just won the job as the starter. His first game he had 4 solos, 4 assists, and 2 sacks. I picked him up after week 1 in like 4 leagues and he went on to win the NFL Defensive MVP and had a very respectable career. Definitely the exception not the rule but for a week 1 waiver wire pick up he became a monster.
Hopefully this gives you a few tips. They don't use words like Average Draft Position but I follow those metrics as well. Show up prepared, look at tiers, and when the meat and potatoes of the draft is done you can find gems that help your defense be great and help you win that fantasy trophy!
The Hellion