If he get's this pick right then I think he's pretty much a lock to win in November, but if he gets this wrong then I think Trump still has a chance.
In my opinion, his safest pick is Amy Klobuchar. She's non-controversial, she's a woman to excite the women, and she's mid-western which should help in the rust belt battleground states which is where this election will be won.
Allow me to also throw out a wild card scenario. I think it's possible Trump replaces Mike Pence on the ticket, possibly with a woman. Trump knows his poll numbers don't look good so he may try to shake things up as he often does.