F17 is ideal but will be very expensive considering you’re on a budget. Do you have a mitre10, Bowens or Home hardware to buy mgp10 pine from ? Be worth driving to Melbourne to get it if you can’t locally.
Built my son’s king single bed frame from F17 - 90x45 posts, 140x45 rails, tongue and groove flooring for bed head infill. Connected the rails to head and foot with ‘bed bolts’. Rock solid. This was about 10 years ago though when F17 was affordable and available for diy projects.