Will i know the gear you are talking about, i would say no. ( i think it is that gear on Gumtree?)
I remember a learner coming to me last summer saying he had just picked up a bargain . The gear was as old as the hills ( early 2000's), and the guy who sold it too him said..
' it will sink up too your knees, and them go like a rocket when the wind blows'- He sold this to the above learner!!!.
My advice is ask guys at the beach, on this forum and double double check before you buy ol gear off someone who has been out of the sport for years..
Size of board really will come down to how fast you want to progress as well,
do you want to take up windsurfing and do it nearly every day this summer---then get something a bit smaller ( a bit of a challenge but you will grow into it).
If you want to progress without any hassles then borrow our Tassie club boards for two weeks ( big starboard starts) and then get a early entry level freeride.
There are plenty of 50kg guys and girls who are happy with there Jp funsters of 145 litres and similar boards- still learning the basics of gybes , footstraps, harness and waterstarts.
And there will be a lot of people lined up behind you to buy it when you are finished