yes i think once you buy it you get unlimited downloads as long as you use th same PSN user info that you used to purchase the game. there was a thread for this type of question befor try searching for it it will give you more details on the subject.
Someone feel free to correct me if I am wrong, but I believe you are allowed to download your digital purchases on 5 different units. I believe you get unlimited downloads on the machine you purchased it on, though I am not sure if that provision extends to the extra 4 machines.
Yes you can re-download them, Just login in back to your old name you bought content on. Look up in the upper-right corner of your screen once you are in the Store. Next to your shopping basket you can see a tab labeled, "Download List". From here you'll see a list of what you've downloaded, and these items will be available to you ro reclaim free of charge.