I think it's a fair price for McDavid, and I'll bet you could get more than you paid for McDavid by the end of the year. So the value is there.
Your goaltending will be weak afterwards, and I don't see a lot of depth in your keepers. I'd ask for a pick or two; or swap picks (his higher [better] picks for your lower [worse] picks). Since it sounds like this was offered to you and he's in win now mode, he just might be okay with offering some picks. Try and get picks with every trade and continue the rebuild with solid youth. McDavid is definitely a cornerstone!