Unless your PC already has some other operating system on it (necessary to get online and d/l Windows 7 to some other secondary drive), you'll need to buy and install an optical drive. Fortunately they are relatively inexpensive; $20-$22 online.
Yes, assuming you have an .iso from the proper version of Windows and a 4GB or larger flash stick, it will be no issue.
Simply follow This Guide and use Microsoft's Windows 7 USB/DVD download tool to make a bootable flash stick.
Then just boot from the USB stick and install like normal.