I'm with CDI. Go for the MK2n especially from a value per dollar spent standpoint.
I picked up a used one in absolutely pristine condition to be the leader to my 30D. The only real frustration is that the controls are set up completely different from the 30D. The MK3 has a control layout like the 30D & 5D.
I can tell you that recently I got the chance to play with the MK3 and a 70-200 2.8 IS and it feels at least half as heavy as the MK2n and 70-200.
Also, I would suggest dumping the 17-55 and picking up the 24-70. You can use the 24-70 on both, but the 17-55 on the 30D only.
Heres a thought...in terms of US$
OPTION 1
-- Use the TC with your 70-200 ------- + $000
-- Sell 17-55 --------------- +$800
-- Buy used low click MKIIn ------------- - $2500
-- Buy 24-70 --------------- - $1150
-- Buy 16-35 II --------------- - $1500
-- Buy 15 fisheye (just for fun) --------- - $580
...............total cost = $4930
Focal length covered = 15 to 280 (24 to 448 on 30D & 19.5 to 364 on MKIIn) if my math is correct.
OPTION 2
-- Buy MKIII --------------- - $4500 to $5000
IMHO - looks like you have the choice of one HELL OF A KICK ASS KIT, vs. 1 new body.
Choice would be easy for me 