KCDW
I find my Dell Axim hard to read in bright sunlight. It's not as bad when I take off the screen protector, but I prefer to leave it on.
It's fine for "following the line" and you can see your waypoints no problem, but I wouldn't be trying to read speeds and eta too often. It's a sunglasses off, hand over the screen and eyes close to the screen job on a sunny day.
Having said that you costs might be a bit off. You already have the PDA, so all you need is a gps. I got one on ebay for €100 new. (It's a cable job, not BT). That's about £65stg.
Ebay GPS
Mounting is a further cost too.
As for PocketFMS which you mentioned, I find it very good as a moving map. Not the best for fiddling with in the air though.
dp