To take a playoff contending team that isn't favorite and make a trade(s) to make them the favorite means one thing....speed
Thats speed at RB, WR, CB or possibly MLB to make a significant impact on an already good team.
That means trading for one of those players in his 4th, 5th or 6th season most likely to get the high end speed you are looking for.
Most bad teams are looking to dump aging vets in there 8th, 9th or 10th season which means they have already begun the decline in the one area you are looking to add.
I would seriously consider trading for a 90 speed RB, WR, CB or 80 speed MLB in year 5 or 6 but most teams with that type of playing have already committed a huge contract to that player.
Anyone else would be of minimal help to pushing a team "over the top". Sure I would trade for a 9th year LT in his 90s but owners want too high a pick for a player that old and its not worth it.