The invalid checksum is telling you that the checksum is not correct for the key you entered. We already know that is the case because the key contains invalid characters. This does not necessarily mean the checksum contains errors - the checksum may very well be the correct checksum for the correct key and will validate just fine once you sort the other errors.
Maybe we need to take a step back and ask some basic questions like why didn't the OP check the private key when he first got it to make sure it was valid?
Use btcrecover as I suggested above. If you are struggling with this, then your only option would be to ask a third party wallet recovery service to run it for you. I'd be happy to give it a shot as well, but this will necessitate you sharing your incorrect private key with either the wallet recovery service or myself.
If the private key cannot be validated then dispute the transaction with paypal get your money back. post on the forum that they tried to scam you. etc. etc. if the private key validates and is associated to an address that holds the amount of bitcoin you purchased then what you do is move them ASAP.