There is a fee for going through the Ausländer Grundbuchverkehrs Kommission and it is based on the property value. I know of at least one person who was charged for this earlier in the year and it takes times too. I also know someone who just had to go through the motions and get a Negativbestätigung and were not charged. Yes, you are right lawyers love to charge fees and a lot of them are unfamiliar with the Withdrawal Agreement.
The Withdrawal Agreement is not explicit about lots of things and property rights is one of them. There are now multiple legal opinions on this issue for Austria and both the British Embassy and Austrian Government are aware of the issue and are working on it. With regards the EU, here is a link published by the EU on Enforcement of Individual Rights: https://ec.europa.eu/info/sites/default/files/enforcement_of_individual_rights_under_the_withdrawal_agreement_v5_en.pdf . Given the recent communication from the Embassy re. property rights I am hopeful that it can be resolved without having to escalate the issue. Unfortunately it is going to take time.
Steve, where are you buying exactly?