Yes you can purchase a key from Microsoft now, later or anytime. Here is a link to the newegg (very reputable online site) products they offer:
http://www.newegg.com/Operating-Systems/SubCategory/ID-368
The OEM keys are cheaper and they work just fine but they are tied to your hardware, meaning if you get a new computer your key will no longer work (for the new computer).
I would go with a 64 bit system over a 32 bit system if you can because it's faster and it can access more ram (32 bit systems can only use up to ~4GB) plus it's the same price so why not.
EDIT: if you buy a 64 bit key, I don't know/think that would work with your 32 bit download but never fear, simply reinstall your windows copy with the one you bought and use the key you bought and you are good to go. Also, here is a download of the files (but no key is given).
http://www.mydigitallife.info/official-windows-7-sp1-iso-from-digital-river/