I've family in Poolewe and Achnasheen so I've played in that neck of the woods a fair bit and am planning on being back up there at the tail end of March.
Depends what sort of mileage you are looking to do on a daily basis but that part of the world is fantastic for walking. I've nabbed lifts on the post bus/van and one of the school coaches doing a run out of Gairloch before, friendly people will help you get where you need to be or a good bit nearer.
The route here
www.greatwildernesschallenge.info/index.asp?pageid=63251 would get you in to Poolewe (camp site on the edge of Loch Ewe with a lovely vista to wake up to and Inverewe Gardens worth a visit if that floats your boat) from a little s/w of Ullapool and from there I'd head along Lock Maree (has an island with a loch with and island inside, a quirk I can't help but like) and camp on the beach at the south end of the loch for sure, I try and squeeze at least one night there if I'm up that way because it's one of my fave spots and not a man made object in sight, perhaps pop up Slioch if you fancy and you're only a short way from Kinlochewe then if you're after some more creature comforts.
If it were me then I'd be finding a route over the hills to Achnasheen then but it's equally beautiful to head further west and back to the coast which it sounds like you may prefer especially if you fancy heading on over to Skye. Whether you go Torridon way or Locharron way you're in for some stunning views.
Feel free to shoot me any questions if I've been to vague on anything and I'll try and help.