Which antivirus would be best for my situation?

My current project requires me to download images which I send to a client through a shared folder on a cloud storage (dropbox). I'm like the average PC user and so not that techie. My desktop got infected by a virus recently (failed to ask the repair guy what kind) and infected the files on my dropbox as well. Now, my PC is clean after having it reformatted but my files in the dropbox website are still there, obviously still infected as they have the ".exe" extension which the infected files on my PC had too, prior to the reformat.

My questions are:
1. Which antivirus would best alert me if ever I download an infected image/file? Some suggestions given to me were AVG, MSE, and MBAM but I'd like to know which one would help me better in terms of downloading images/files since I can't check each website that I download images from.

2. I need to reinstall the dropbox application to my desktop but I'm afraid I might get the virus again if I'm not that careful or possibly some form of syncing happens with the infected files that are still in the dropbox website. Would the same antivirus help me get rid of those virus from the files that are still up on the dropbox website? How do I do that?

3. I believe the free version of an antivirus have limited features, so is it better to buy the paid version and skip trying the free one? And is it advisable to have more than 1 antivirus installed on your computer?

Btw, I use Win7 if that's any importance.

Thanks.