Since Adventure mode, as well as your plans give you a small measure of control and “targeting” of legendaries, here are some things you can keep in mind.

As a loose guideline, there are items that can go into trifecta and then there are the rest, which tends to be stat sticks. For the stat sticks, generally, legendaries will outperform with a simple roll. For the potential trifectas however, you will be sticking with the yellow for a while until you get a well rolled legendary.

Weapon: Weapon is the highest determinant of your dps, so you will want to fill this up with a legendary as fast as possible. At the minimum you want a high dps, along with a socket, and your main stat. You can add on ias or vit or legendary exclusive affix for the final. For 1h users, Mad Monarch Scepter is a semi targetable drop by farming act 1 adventure caches. 2h users can try act 3 for burst of wrath (the resource axe). Some plans, such as the set, will also allow you to craft a legendary weapon (at a steep cost).
Rare damage will run 1k+ to a little over 2k for a 1h, while a leg 1h usually starts from 2k+.

Offhand: This is a matter of luck (unless you are a crusader then some shields will drop in adventure), but you can always buy orbs off Kadala for 5 blood shards! Some offhands will change the build immensely.

Trifectas: Generally a rare will suffice and outperform any non well rolled legs here
Gloves, Amulet, Rings
Glove: 600 main stat, 10 crit, 50cd, 6ias
Amulet: 600 main stat, 10 crit, 100cd, elemental+%/vit/6ias
Rings: Main Stat, 6 crit, 50cd, vit/ias
Generally these slots require at least 3 good stats, with you having to reroll one of them into perfect or rolling out a fourth stat. It’s a matter of chance whether the legendary will drop in a favorable way and many a times it does not. The extra 100 stat/10ar/200armor it can provide usually is not enough to cover for say 10 crit or 100 crit damage. Certain situational exceptions are possible however such as Shrine Gloves (Adventure Mode) that gives 10 minutes of shrine effects (1 hr as bug), Amulet that absorbs entire element (e.g. arcane), Ring of Royal Grandeur (reduces set requirement by 1)

Holey Parts: The prevalence of sockets on these outstrip the bonus unless your legendary rolls those sockets.
Chest and Pants, 3 socket and 2 socket
Each socket can basically give you 280 stat, compared to the slight boost of a legendary. Rares will provide you more damage until you get a legendary that rerolls into sockets.

Randoms: Legendaries are usually better here, unless you want to roll something specific
Bracers, Head, Shoulders, Boots, Belt
In almost every case here the legendary will be better! Bracers is particularly easy (and cheap) to reroll and you should have a 400main stat 400 vit 6 crit and +dmg ele/skill/ All resist/Armor very soon. Many of the bracers will allow you to swap out one of the primary affixes for say ias (increased attack speed) or special effect (nemesis bracers spawn elite at shrine, trag oul has 46s cd removal on healing shrine, reaper wraps resource health globes etc) and even better sometimes they take secondary affixes instead.

Heads and Belts have the irony that a crit mempo and well roll witching hour will be BIS for a very long time until they are replaced by a very well done leg, often of the same type. This is due to the 9ias on mempo and 50cd 9ias potential on the witching hour. If you don’t have this however, this two slots have no particular specialty that they won’t be replaced by a legendary! (the new legs do roll 6-7ias at 70, lower than 90, for helm a leg might not offer you a socket however)

Shoulders, Boots
Very clear cut legendaries, movespeed can be paragon levelled!