Depending on your grading scale a rough guide as follows.
The inventions are around Grade 4/5
Sinfonias 6/7
WTC is invariably 7 or 8 if you're doing both the P and F. ...as J_menz noted, there are some easier preludes.
Learn each voice separately in the sinfonias, as opposed to learning separate hands. Then piece them together as combinations of voices, rather than just RH and LH together.
Like This -
Learn to play voice 1 together with voice 2,
Then learn to play voice 1 together with voice 3
Then learn to play voice 2 together with voice 3
Then learn all 3 voices together.
Also, bernhard may have posted a complete learning order as per the way bach taught them, he did for the inventions - do a forum search.
EDIT: I would hasten to add that the grading I presented there was very generalised and is simply as per this websites music collection..
You may also find threads such as this one interesting -
https://www.pianostreet.com/smf/index.php/topic,4736.msg44774.html#msg44774