A woosh test can only be run on an ADSL activated line as far as I can remember so if you don't have ADSL PlusNet can't do a woosh test.
There are no longer any limits for a 512K connection, 60dB for 1Mb and 45dB for 2Mb. BT will will attempt to get 512K working on any line and will even do a home visit to change your master socket to get it working.
The only way to find out if you can get ADSL is to signup / upgrade to it and see what happens. In the unlikely event of it being refused you will get you activation fee and any other payments back.