In regards to the best timber for a study desk, I would choose something slightly denser than pine. If you are writing on paper, directly on the desk, you can leave tracking marks in pine quite easily as it is a softwood. Beech, Oak, Hevea and Ash are all considered hardwoods and would be more than suitable for the desk.
Bunnings panels come finished and ready to use. There should be little to no sanding required before you apply your stain. For best results, the application of the stain will be done as per the manufacturer's specifications. I would advise applying several light coats and build the colour up to your desired strength instead of one thick coat. Feast Watson has a great range of stains to choose from and I have had success using them in the past. You can then apply a polyurethane coating if you would like a hard coat to prevent wear and tear on the surface.
Project panels are small pieces of timber glued together. You can see the growth rings on this 600 x 18mm x 1.2m Pine Utility Panel which would be more suitable. You would need to build the coats of stain up slowly, as you will cover over the rings completely if you use a heavy coat. To get something with a visible grain effect you would need to use solid timber and not finger jointed project panels. - MitchellMc