Whoa - back it up a minute.
Have you got sharpening stuff - other than a 1 to2 hp grinder? Even if the iron comes dead flat on the back - at least the front inch or two, sharpened - and honed - at some point it will get dull.
If the back of the iron ISN'T flat it needs to be made so - and THAT can't be done on a grinder. So - you've got Scary Sharp (tm) (float glass with sandpaper, preferably wet/dry, stuck to it - in eight or 10 grits), the Diamond Plates, India/Arkansas/Japanese stones and mayybe a leather strop. Then you probably will start with some sort of sharpening jig to hold the iron/chisel at the proper angle to the sharpening surface. When you've acquired the sharpening stuff you'll discover that it's cost several times what plane you started with. Of course, when you distribute the sharpening stuff cost over - oh let's say 8 to 10 planes, and a couple of sets of bench chisel, the cost/sharp tool starts dropping off to reasonable. If you slide ALL THE WAY DOWN THE SLIPPERY SLOPE, you won't even recall spending ANY money on sharpening stuff.
My vote for a first plane is a block plane - or better yet - the LN Block/Rabbet plane. Great for tuning drawer fit, can be used to tune a tenon, including a shoulder if you're careful - and it's pretty to look at - if a tool can be called pretty.
Second plane - a #5. Good for joining the edge of shorter stuff, taking out a bit of cupping or bowing - on shorter stuff, thinning down narrow parts, etc..
Did anyone mention that when you jump from a block plane size to the #4 and above - you'll probably need a vise, and a bench to hold it on - oh and "dogs" - with dogholes - you need to keep the piece of wood you're planing from moving around.
Oh - and if it's ply you want to work with - BELT SANDER. Ply is REALLY hard on handplane irons - can even chip your edge.
charlie b