My winter indoor hydroponic/late crop grow log.

[hollohas]

How warm do tomatoes & lettuce require it to be? We don't exactly crank up the heat in the winter but have more than ample sunlight in an unused room.

Tomatoes originated in Central America so they generally prefer it warmer. But from the below I think you'd be fine in a cooler house.

From the Burpee website.

In general, tomatoes will stop producing fruit when temperatures drop below 50°F or rise above 90°F.

And there are varieties of lettuce for warm and cooler temps. Iceberg for example prefers cooler temps.