Welcome to the forum.
Start by updating your profile so folks can make geographically correct referrals for material - doesn't do you much good to know that everything you need is in a surplus store in California for instance.
Mega has 69 IO pins. Your list doesn't appear to need that many. The touchscreen does use a few. Looking at the pictures it appears that at least 32 pins are not committed at all. If your other components needs I2C or SPI access, you may have to add some jumper wires to bring the two I2C pins and the three SPI pins to an accessible point.
Or make the touchscreen cards the last thing on your stack of parts.
Your sketch can include a clock program, may drift a very little bit, you can update it when connected to the PC as suggested.
You can connect to the internet with an Ethernet shield. Probably get the time there too.
You can get Wifi, bluetooth, 315/434 MHz, 2.4GHz, all kinds of ways to connect wireless to your PC.