Hi Jase. Do you have a satnav? And do you have plenty of gears,preferably a compact or a triple?
I don't live there now but grew up in Powys and though it is amazing cycling country it is pretty hilly and lots of the minor back roads are lacking in signs. There are lots of drovers roads that don't bother gradually climbing the hills like in France but go directly up the steep bits, gradients of over 15% are common. Road surfaces can be a bit sketchy too so I'd opt for fatter tyres if you can, at least 25's to make it a bit more comfortable.
Phil sounds like he knows the Shropshire side very well so I'd heed his advice and definitely avoid the A5. If I was you I'd get the train to Shrewsbury or Welshpool and then go from there. Its less than an hour on the train.
Here's a hilly route I've just cobbled together from Shrewsbury to Machynlleth that follows some of Phil's suggestions and mostly avoids A-roads.
http://ridewithgps.com/routes/4195253
Check out the profile of the finish, pretty much downhill all the way for the last 8 miles.
The A470 and A458 look OK on the map but are very busy roads and not fun to cycle. If you are into amazing scenery I'd highly recommend the road over the top from Llanidloes to Machynlleth on the above route. Its one of my all time favourites. On a nice day it has jaw dropping views, it may be a bit lung busting but its totally worth it. Check out some of the pictures of that section on this site:
http://www.gps-routes.co.uk/routes/home.nsf/RoutesLinksCycle/machynlleth-to-llanidloes-cycle-route.
If you wanted more hills I'd also recommend heading from Forden up to Montgomery then up and over Cefn y Coed, from the top its downhill all the way to Abermule where you can pick up the route I've plotted above.
I'm not sure of the section from Mach to Aberdovey so I've not done that bit. I do recall a route I did years ago that avoided the A-road and went through Happy Valley, that was great but can't find a route for it.
Anyway just my 2p worth. Have fun.