My advocate tells me that its important to have it witnessed by an advocate,but not compulsory. What your lawyer may have meant by 'stamped' is stamped for nominal duty at the lands office. This is highly adviseable as it makes the agreement 'admissible evidence' in the event that the matter ever lands in court. Put it this way,if the matter lands in court and you require to submit the sale agreement as evidence,then it needs to have been stamped to be admitted as evidence,in order to show that the agreement was actually made at that date. Secondly,if you are applying for a caution (which you lodge e.g. the seller sells to you and then you discover he is selling to someone else),then a sale agreement that is not stamped cannot be relied on/is not enforceable and so you wont get the caution registered by the registrar. There is a high court matter where the judge said where the sale agreement is not stamped,even though it was not admissible as evidence at the time,the court can still allow you,if you apply,to go and have it stamped. But note that since assessment for stamping is based on the date of agreement,if more than thirty days have expired then there will be penalties to pay. So its highly adviseable (and relatively cheap,considering the benefits) to have the document stamped at the word go.
Hope this helps