I considered the 7D vs 7DII. The II is better built, 2fps faster, half a stop better in low light, focuses at f/8, has a superior AF tracking system, contains dual card slots, provides improved video options, and has many additional minor improvements.
I then checked sample images taken with both bodies and quite honestly couldn't tell which was which. Within most situations they will provide almost identical pictures. Keeper rate and focus accuracy will of course be slightly better with the 7DII, but at 8 or 10fps and using identical lenses you'll probably get some good shots with both.
I purchased a used 7D Classic as a mate for my 6D and do not regret the decision. The 7D provides me with range and near fps of the 7DII, whilst the 6D gives low light performance and ultimate IQ vastly superior to it. Both my 7D (used) + 6D (new) together cost the same as a new 7DII, and I can pick and choose which body is best for whatever situation. For outdoor sports or the odd bird I choose the 7D. For anything indoors, low light or landscapes I take the 6D. Of the two the 6D gets used most because it's IQ tends to outweight faster AF and higher fps for family and holiday snaps, which is my most common usage.
I would of course prefer a 7DII over my 7D, but the extra £600-£700 won't give me much. On the other hand I would keep my 6D over a 7DII.
If I was you I'd either keep the 1D and add a used 7D Classic, or sell the 1D and buy a 6D + 7D Classic. Best of both worlds.