G'day Grassy,
Ahh a scuba diver, talk about a veritable minefield of techno goodies to keep your wallet empty.
As you've said, proper planning is the key. I don't have a GPS, used a few (I let the boss buy them). GPS's are nice and useful but not essential.
If I were you, I'd make a list of what you want the GPS to do and cost (eg, colour, moving map, updatable data base, portable and so on). Then pour over glossy brochures, search online and read reviews in magazines. (Head off to the library for them).
Reduce your list to a couple of units. Now go and have a look at them. If you can, ask around your flying club/school for demo's of the units and people's recommendations. Have a go at them yourself. Model X might do exactly what model Y does but to you, in a more intuitive way. You're the one that has to use it, you want one that will fit your needs.