Napalm looked nice, but I think hand crafted pixels rather than rendered graphics could look better.
My advice would be aim for 68000 and 1mb chip, it will teach you how to code tightly, and well. Also, you'd be aiming for the biggest audience (hell, even my phone and dreamcast can run that spec on an emu!)
But remember you are writing in basic, so your never going to get top speed unless you can implement some inline assembler.
I'd start simple, just getting your guys moving on a tiled back ground, and work up from there.
You could always increase the spec later in the project, or for a later version.
Obviously there are lots of sites with ripped sprites and tiles you could use as stand in's. Once a graphics artist sees your engine running, i'm sure they'd jump at the chance to work with you!
at a minimum, id say it would be nice to have: group select, Attack guard and waypoint, base building, repair units, fog of war, veteran units, and a nice storyline