You can buy the upgrade, but it's cheaper to buy the full version, usually, if you shop around.
Yes you can. The upgrade and full product versions are identical so buy whichever is the cheapest as it’s not always the upgrade on some websites.
PSE11 is due to be launched soon (normally in September) so you may wish to delay your purchase to get the most up to date software - unless you can get a great discount on the current version and can‘t wait.
To expand on what Barbara and 99jon have said, there's no such thing as an "upgrade patch" to update an old version of PSE to a newer version. Each version of PSE is a self-contained, standalone, full product.