And in case of wind drop (which sometimes happens on our lake) will I sink???
No, you wont sink. As a very rough guide 1ltr of volume will support 1kg of sailor plus you need to add another 20kg for your rig and wetsuit. That gives you plenty of room with a Go139.
It is also a very wide board which aids float and stability so you should find it ok. The Go range are suprisingly good and very good value for money. You will have a lot of fun on one.
Before you gave up, what was your standard of sailing? If you were planing in the straps and gybing then a Carve might be another option. They are a higher performance board and hence cost more but are lighter and more agile, could be fun. The Go is built to be a bit tougher and will take more abuse though.