Planted tanks can be a challenge but are worth the effort. You can put sand over flourite but you will find the sand will migrate down into the flourite and when you plant new plants, you will have the flourite showing. If you want sand, go for it and plan on using plant tabs and skip the flourite. I went to fine river gravel and flourite mixed, mostly to hold the price down and still have the benefit of flourite.
I would go with the biggest tank you can get. It enables you to have some of the plants that grow larger.