Install Chamberlain garage door opener on 8 ft door 

The belt drives are typically more expensive. If you got for the same price that’s great.

Same question I have

Find the extension sold in the store or on line. Check if old opener and new one use the same extension, if they do, you can buy your new opener and use the extension from your old opener.

A standard opener won’t work for a 9’. Not just the extension but the chain won’t be long enough either. I’d have to call to a vendor for that one.

Standard opener works fine for a 9'. They make 10' extensions for that. They are harder to find though because less common.