Whether you need padded tights will depend a lot on how well padded your saddle is. There is such a thing as too much padding. I use inexpensive leggings from the active wear section on the bike on a regular basis and often hit 100km in a ride. I have several that I've had for several seasons. For cold, my greatest concern is my core temperature. To deal with that, I use a thin "puffy" (polyfill, fiberfill, down if you want to spend for it) vest without sleeves. This will handle any cold you run into, and I have a wind/waterproof outer shell for more inclement conditions. Remember: if your core is warm, any blood going to your extremities will be warm. If you bundle up too much, you will sweat, and that will make you colder (that is what sweat is *for*, after all).
keep your hands warm (to keep from cramping), and keep your core warm. The rest will take care of itself as long as the wind is at least deflected. If your riding conditions are going to vary a lot, you can carry arm and leg warmers, which can be pulled on while on the side of the road if needed. If your longest ride is on the order of 30km (20mi), then you should not need to spend a lot on special kit for cold on the bike. Just need to keep most of the wind off.
Disclaimer: this advice should work fine to low 40's(f) to high 30's(f) without a lot of wind chill off the bike. Down below the frost and ice point, heavier bundling on your extremities will be needed.