So you might not have the ideal personnel yet, but you want to end up with a base 4-3 set.
Defensive Line - First DT needs to be the best run stopper you can get. Second DT needs to be balanced, good block shedding and power move ability. Both DEs need to be pass rushers.
Linebackers - MLB needs to be a good coverage guy. Almost like a hard hitting strong safety with the tackling power and size of a MLB, how Brian Urlacher was. I like Sam (strong side OLB) to be balanced; good zone coverage but still a good tackler. Will (the weak side OLB) should be a coverage guy first and foremost, because he will likely be covering a slot guy when the offense spreads it out. Overall, always look for speed with your linebackers.
Safeties - I would employ coverage FS types in both safety positions. Heck, even balanced cornerbacks if you can get them. They are mainly to take care of anything deep, and are the last line of defense, so speed and zone coverage are the most important traits they need.
Cornerbacks - Yes, hard hitting types. Tackle, hit power, and press are your main concerns as they won’t be asked to run alongside a speedy receiver going deep very much at all. They need to wreak havoc on short passes, toss plays, outside runs, and passes to the flats, and hopefully create turnovers from it.
Also consider awareness for all positions when you can’t decide between two prospective recruits. You always want them to be in the right place in the right times.