Hi nereid, welcome to the forums.
Here is a comparison chart between the Home and Pro versions.
The detection and cleaning abilities are the same. Some of the features only available in the pro include automatic (user-set) handling of detections, script blocking, scheduled scans, and more timely updates for recently discovered threats.
I would purchase the pro without hesitation if those features were useful to me. Because I like to interact with the current state of my computer security, they don't matter so much, although would definitely be of use.
Regarding this specific detection, it sounds like the webshield component of Avast prevented the download occurring, after detecting a malicious script. I've placed two pictures below. One is a sample webshield detection, the other an on-access (standard shield) detection.
Some names have been edited out by me, but there is enough on the pictures to tell one type from the other. Do you remember which of the warning messages you saw?
If it was the webshield detection, the connection was terminated before anything actually downloaded. If the standard shield detection, something downloaded to your temp. internet files, and was then caught by Avast, and quarantined.
For peace of mind, you may wish to perform another scan by a good quality demand scanner. There are free versions of
MBAM and
Superantispyware available. Both are excellent demand scanners. Grab yourself a second (or even third) opinion.
I also recommend you clear your temporary internet files and perform a disk cleanup on a regular basis.
I can't actually answer the questions 1 and 2 you pose with any authority, but regarding (1) I don't see why it shouldn't be possible to download the pro (trial) version, uninstall the home, and install the pro. It may even be possible to install the pro over the home, but I've never tried and don't know.
I would still be reasonably confident of banking transactions if this were a webshield detection and the connection aborted. If it were a standard shield detection, the computer appeared to be behaving normally, and either of the two demand scanners found nothing significant, I'd be confident, too.
I would recommend you use a two way firewall if you don't already. This is a good way to find out if something new on your computer tries to connect outbound. Most malware is pretty useless to its user if it can't connect outbound.