Buy this one if you can...

TX60

My buddy has it and loves it. Even alerts his smartphone when the temp drops below a certain threshold. I want to get one when/if I have money ever again:)

Something like this, while more expensive, also does a ton of data collection and is accessible from the internet...

Ambient weather

My weather station is the one that Franktank posted that run with Cumulus software but it would be it would be an arkward match for your needs.

If you want to track minimum and maximum only then most the ones at places like Walmart should work but you would have to track the data manually.

Now if you have a bunch of spare coin them Davis makes a $500+ unit that you can buy extra probed for.

If just knowing the coldest it ever became a min/max greenhouse thermometer should do the trick without spending lots of cash.

Frank what a great site thanks. My husband is a weather bug and love gadgets too boot. You've just taken care of his Christmas present. Mrs. G