IO540
For balance, my experience of rack mount vs handheld is exactly the opposite of yours.
I have found handheld units to be unreliable (I had a Tracker II that just kept freezing) and much more likely to lose signal. My plumbed in Garmin has never once lost signal or RAIM in the many years I have owned it, whereas whenever I fly with people with handhelds, including Skyforce IIIc, signal has been lost at some point, presumably when the airframe eclipses the satellites from the aerial.
On the wider point, I agree with the general concensus, get the DR and pilotage skills up to a point where you can rely on them, then get a good, moving map, GPS. I recommend the Skyforce IIIc, as it has much more detail than most, and is easier to read. When you get your own aircraft, get a proper aerial and plumbing.
W