It would safe Blizzard hell of a lot of work to combine these two.
Ladder should be self found only, no auction house, credit card dual wielding or trading with bots.
A race lasting about 3-6 months should be fine.
-Once the race is over the top 100 will be given a badge on their accounts. This shows the highest rank you've achieved in ladder mode. You can keep improving this badge by gaining a higher rank.
-Characters serve no need to transfer to normal mode once the race is over for these reasons:
a.One class can respec infinitely, no need for double chars.
b. The biggest reason people would want this is to transfer high end items to normal mode. This may never happen!
optionally place a max amount of playtime to avoid bots winning the race by being on 24/7. 4 hours a day should be fine, after this you can continue with your other chars. Face it, to beat bots there needs to be some kind of regulation.