I've for one had the same thought.
For Tools, I'd recommend the following:
Translhextion - You're best bet for a hex editor.
Tile Molester - Best Graphics editing utility ever
Temporal Flux - Editing events, decompress text, changing text, exporting, decompressing graphics, etc...
SNESPAL - Changing Palette
There's probably more, but I can't remember quite now.
With a team of 3-4...I'd expect a year to 4 years if you're working really slow. If you guys constantly work on it, probably a year to 2 years at tops. It's a tough thing to do, but don't let time discourage you. Just keep going at it until you get what you like.