DO NOT apply any seed until at least 2 and I strongly suggest 3 weeks after you spray RoundUp. The pesticide label will tell you this. RoundUp is in the soil and won't break down for 2-3 weeks and can kill any seed you put down afterward. Putting lime down actually raises the pH level. This will knock back the moss, but I would suggest getting a soil test done. You don't want to put too much down. Assuming you don't have a pH level that is way low, you might need 20-40 pounds of lime per 1,000 square feet just to raise the pH level half a point (6.5 to 7.0).
What part of the country do you live in OP? What kind of grass do you have right now? If you have a bunch of bare spots, seeding even after aerating might not be enough. You will want to put 1/4 inch of topsoil on top of the bare spots. Don't water as needed. The first day you seed, the seed needs 20-30 minutes of sprinkling. Then just keep the soil moist for 2-3 weeks. 5 minutes of watering or so per day or as needed if there is no rain is recommended.