Hello and welcome to the site. Thank you for your question.
Ideally, you should have proper documentation to support the purchase price.
If you are able to prepare a schedule of costs with supporting documents (the more the better) comprising of proof of payments i.e bank statements, credit card payments, cash withdrawals for spend etc, it would be helpful.
In the absence of it, you would have to defend the figures.
Having said that, this would only be required in the event there is an enquiry from the the Tax Office.
Your starting point is to report the gain based on whatever documentation you have in your possession and take it from there. Your tax return may not be selected for scrutiny and/or enquiry.
I hope this is helpful and answers your question.
If you have any other questions, please ask me before you rate my service – I’ll be happy to respond.